The Affective Filter Hypothesis is one of five major hypothesis for second language learning proposed by Stephen Krashen in his 1982 book Principles and practice in second language acquisition.
Affect is defined as outer emotional expression. However, within a cognitive context, affect is re-defined as a combination of environmental and biological variables that affect the emotional state of the student.
According to theories of learning and motivation, learning is most effectively acquired when students are in an environment that is non-threatening, secure, and where they feel successful and allowed to make mistakes.
A low-affective filter- refers to a clear and relaxed emotional state. The student is free from stressors, fears, or anxiety for learning. This entails that the student is ready to filter in information, because the state of mind is at an optimal level for learning; a student that is not biased and does not feel threatened will listen better, establish connections, and build the schema that is so necessary to decode and encode in SLA.
A high-affective filter- refers to an over-stimulated and heightened emotional state, usually rife with stressors, fears, and anxiety. A student that comes from a very dysfunctional home, who has prejudices against learning L2, or who feels incompetent enough to try is said to have a "filled" filter, or a "high filter". This means that the stimuli which pervades the student's emotional state blocks any learning opportunity.
The best way to illustrate examples of low and high affective filters is to look at how smaller children acquire first and second languages at a much higher speed and with much more accuracy of use than older students.
Younger children, free of prejudices and bias (functional children, that is) possess the motivation and the open mind which is often associated with a low affective filter. Older learners may have pre-conceived notions about their own learning abilities that may hinder their learning chances.
Therefore, teachers of L2 must keep in mind that the learning environment must appeal to the student, lower their affective filters, and make the lesson relevant, appealing, and based on the student's level of ability. Other than that, it will be a lost teaching moment