The Affective Filter hypothesis was developed by Stephen Krashen in 1982, and it is part of a total of five different hypotheses regarding the acquisition of foreign and second languages.
The purpose of the hypothesis is to help teachers of foreign languages and L2 to understand the difference between language learning and language acquisition, and to avert educators about the "attitudinal variables" that may affect student success in SLA. This way, new interventions for SLA can be created as well as best practices that will prove to be more effective.
Krashen cites "attitudinal variables" as the causative factors of success in the L2 environment. These variables respond to the number of outside influences that may render a student scared, or too stressed, to assimilate the target language.
For example, in theory, the student's level of affect/emotion (or, the filter) will be lowered (free from stress and anxiety) when activities are non-threatening nor attempt to challenge the students outside of their current or proximal zone of development.
On the other hand, tasks that are too complex or a vocabulary that is introduced using a bad teaching practice without context will "fill" or "raise" the filter thus blocking the student from learning anything. Therefore, all tasks performed in the L2 should involve the input plus one, ( i +1) method, which is to teach the language and challenge it up at notch one step at a time.
Aside from pointing out the importance of the proper affect level as a preliminary factor for successful L2 learning, the Affective Filter has a further function of differentiating the term "learning" from the term "acquiring" when it comes to new languages.
In the hypothesis, "learning" is alluded to as a passive process; the student often does not initiate the learning process, and is often "spoon-fed" the language over and over. As a result, the student may hear a word being repeated a number of times and end up remembering it. This does not mean that the student has finally "learned" the language, but that he can recall some of it through memory.
Contrastingly, when a language is acquired it means that it has been assimilated, processed, and reproduced through a number of different tasks that can range from writing it, to drawing it, to singing it, or through creating projects with it.
The conceptual framework of the Affective Filter hypothesis is based on the cognitive platform that learning and motivation must go together in order to achieve success. A lesson given without purpose, enthusiasm, or relevance will lead nowhere. This is why it is so important to maintain students in the "low" affective side by providing them with quality education that takes them somewhere.