Children learn to understand words and sentences gradually over time. This starts when they are still babies, for example when they begin to notice that their parent or caregiver says the word ‘bath’ every time they turn on the taps. Over time, as they hear more talking, children start to understand more and more words and sentences. Children need to hear a word many times in lots of different situations before they fully understand it.

https://speechandlanguage.org.uk/educators-and-professionals/resource-library-for-educators/support-young-childrens-understanding-of-words-and-sentences/