Language acquisition is the process of learning how to speak and use language. Think about when you were a baby and did not know how to talk yet. You had to learn how to make sounds, like "ma" and "da", and then eventually you learned more words like "mama" and "dada".
When we're little, our brains are like sponges and they soak up information really easily. We learn a lot about language just by being around people who speak to us, like our parents or caregivers. They talk to us and we listen, and our brains start to understand what they're saying and how to make similar sounds.
As we get a little older, we start to learn more words and sentences. We might ask for things we want, like "milk please" or "more toy". We also start to learn grammar, like putting words in the right order and using the right verb tense (like saying "I will eat" instead of "I eated").
It takes a lot of practice and repetition to get good at speaking and understanding language. We learn by hearing and using language over and over again. This is why it's important to talk to babies and toddlers a lot, even if they don't understand everything yet - they're still learning and processing all the information they hear.