“Classrooms that encourage low filters are those that promote low anxiety among students, that keep students ‘off the defensive’ (Stevick, 1976). …. The Affective Filter hypothesis implies that our pedagogical goals should not only include supplying comprehensible input, but also creating a situation that encourages a low filter.”
“The effective language teacher is someone who can provide input and help make it comprehensible in a low anxiety situation.”
-page 30 -32 of Principles and Practices in Second Language Acquisition by Stephen Krashen
I urge teachers to keep this in mind as they raise their own levels of anxiety trying to learn to teach without classrooms, without walls, without materials, without face-to-face contact in a situation that causes anxiety even for adults. The levels of anxiety in language learners right now are not conducive to language acquisition, no matter how amazingly you teach.
We know more about brain research now than we did in the 70s. Basically, we’re being chased by lions, and our brains are dumping adrenaline and cortisol. We are 100% incapable of overriding a brain telling us we’re being chased by lions and forcing it to learn French.
It doesn’t matter how good of a teacher you are. It doesn’t matter how well you teach right now. 
It matters that your face, your heart, your smile reassures as many students as possible that they are not being chased by lions.
If you’re being required to teach languages right now, remember that brains cannot acquire languages with a high affective filter. When you’re being asked to achieve an impossible goal, remember that some things just don’t apply to languages. Many things we are asked to do, we know do not apply to language acquisition.
The goal is not to teach languages. The goal is to create an environment in which the lion us taking a nap and your students to have a short break from stress.
We’re here to help as much as we possible can.
We understand that your brain also thinks you are being chased by lions, and we will do everything we can to give you a lion-free environment.
“5. THE AFFECTIVE FILTER HYPOTHESIS The Affective Filter hypothesis states how affective factors relate to the second language acquisition process. The concept of an Affective 30
Filter was proposed by Dulay and Burt (1977), and is consistent with the theoretical work done in the area of affective variables and second language acquisition, as well as the hypotheses previously covered in this chapter. Research over the last decade has confirmed that a variety of affective variables relate to success in second language acquisition (reviewed in Krashen, 1981). Most of those studied can be placed into one of these three categories:
(1) Motivation. Performers with high motivation generally do better in second language acquisition (usually, but not always, “integrative”13
(2) Self-confidence. Performers with self-confidence and a good self-image tend to do better in second language acquisition.
(3) Anxiety. Low anxiety appears to be conducive to second language acquisition, whether measured as personal or classroom anxiety. In several places I have hypothesized that these attitudinal factors relate directly to acquisition and not learning, since they tend to show stronger relationships to second language achievement when communicative-type tests are used, tests that tap the acquired rather than the learned system, and when the students taking the test have used the language in “acquisition-rich” situations, situations where comprehensible input was plentiful. The Affective Filter hypothesis captures the relationship between affective variables and the process of second language acquisition by positing that acquirers vary with respect to the strength or level of their Affective Filters. Those whose attitudes are not optimal for second language acquisition will not only tend to seek less input, but they will also have a high or strong Affective Filter–even if they understand the message, the input will not reach the part of the brain responsible for language acquisition, or the language acquisition device. Those with attitudes more conducive to second language acquisition will not only seek and obtain more input, they will also have a lower or weaker filter. They will be more open to the input, and it will strike “deeper” (Stevick, 1976). The Affective Filter hypothesis, represented in Fig. 2.2, claims that 31 the effect of affect is “outside” the language acquisition device proper. It still maintains that input is the primary causative variable in second language acquisition, affective variables acting to impede or facilitate the delivery of input to the language acquisition device. The filter hypothesis explains why it is possible for an acquirer to obtain a great deal of comprehensible input, and yet stop short (and sometimes well short) of the native speaker level (or “fossilize”; Selinker, 1972). When this occurs, it is due to the affective filter.
Fig 2.2. Operation of the “affective filter”. The “affective filter”, posited by Dulay and Burt (1977), acts to prevent input from being used for language acquisition. Acquirers with optimal attitudes (see text) are hypothesized to have “low” affective filters. Classrooms that encourage low filters are those that promote low anxiety among students, that keep students “off the defensive” (Stevick, 1976). This picture does not diminish, in any way, the importance of affective variables in pedagogy. The Affective Filter hypothesis implies that our pedagogical goals should not only include supplying comprehensible input, but also creating a situation that encourages a low filter. As discussed in Chapter V, several methods focus on just this (e.g. CounselingLearning and Suggestopedia). The input hypothesis and the concept of the Affective Filter define the language teacher in a new way. The effective language teacher is someone who can provide input and help make it comprehensible in a low anxiety situation. Of course, many teachers have felt this way about their task for years, at least until they were told otherwise by the experts!”
This books, Principles and Practices is free on Stephen Krashen’s website