The document discusses Krashen's Affective Filter hypothesis, which is one of the five hypotheses that make up his Theory of Second Language Acquisition. The Affective Filter Hypothesis proposes that affective variables like motivation, self-confidence and anxiety play a role in second language acquisition - learners with high motivation, self-confidence and low anxiety are better equipped for language learning success. These affective factors can raise learners' affective filter, preventing comprehensible input from being used for acquisition. The document emphasizes the important role affect plays and the need for teachers to ensure students' affective filters remain low to facilitate learning.