Internalisation is the process of learning something so that it can be used as the basis for production. Once language is internalised, it can then be retained and retrieved when needed for communication.

Example
A learner finds that using a spidergram helps them record a new lexical set on words that describe emotions effectively and so internalising this new vocabulary is easier.

In the classroom
A learner discovering for themselves what the meaning of a new word is by using context will internalise this information more deeply than a learner being told what the meaning is. This kind of processing is key to techniques such as guided discovery, learning strategies such as the example above, and task-based learning.