As we learn, we hypothesise rules, and use these to communicate until we notice that the rule is different, or has exceptions. This leads to classic early mistakes such as 'I seed' and 'I buyed'. Language learning programmes that immerse learners in the target language aim to create the conditions for acquisition to happen in second language learning. In a communicative classroom, opportunities for natural acquisition are often provided alongside opportunities for formal learning, to make the most of both ways of learning.
Example
A learner can acquire language by living in a country where the language they want to speak is used. They do this without formal training and by being in contact with it and needing to use it.
In the classroom
Learners can be encouraged to acquire language by exposure to authentic spoken or written language and authentic communicative tasks, such as watching TV in order to summarise what they understand, rather than to analyse the language they hear in depth.
Further links:
https://www.teachingenglish.org.uk/article/child-a-learner-1