More specifically, it is the name of the theory of the process by which we acquire - or pick up - a second language. This is mainly a subconscious process which happens while we focus on communication. It can be compared with second language learning, which describes how formal language education helps us learn language through more conscious processes.
Example
A learner studying in an English-speaking country may have more success due to the language they acquire in their part-time job than with the language they learn in their class.
In the classroom
Implications for the language classroom include the ideas that the teacher can create contexts for communication which facilitate acquisition, that there is a natural order of acquisition of language, that there are affective filters which inhibit acquisition, especially for adults, and that comprehensible input is very important.
Further links:
https://www.teachingenglish.org.uk/article/what-sla-research-good-anyway
https://www.teachingenglish.org.uk/article/teaching-reading-esol-classroom