Stephen Krashen's Acquisition-learning hypothesis is the most important aspect of his theory of second language acquisition. It states that there are two independent ways in which we develop our linguistic skills: acquisition and learning. According to Krashen acquisition is more important than learning.
Acquisition of language is a subconscious process and the learner is unaware of the process taking place. Once the new knowledge has been acquired, the learner is actually unaware of possessing such knowledge.
Learning a language involves formal instruction and is therefore a conscious process. New language forms are represented and possibly contrasted consciously by the learner as "rules" and "grammar". These "rules" - while known by the student - may well nave no actual impact on the language produced by the student.
A good example would be the third person "s" - a structure "learnt" in the first few weeks of any English course but frequently not "acquired" until very much later.
Implications for teachers
It is clear that as teachers we want to maximise our student's opportunities to acquire language. Consequently, if we accept the hypothesis then we need to spend more time using real language with our students as opposed to teaching them explicit grammar rules.
On the other hand, many students equate learning grammar rules with learning the language and attempting to re-focus teaching away from this method may meet with resistance.