Okay, let's imagine that you're reading a book that has a bunch of sentences in it. When you read each sentence, your brain has to do a lot of work to understand what it means. This is called sentence processing.
First, your brain has to figure out what all the words in the sentence are. Then, it has to figure out how they fit together to make a complete idea. For example, if the sentence says "The cat sat on the mat," your brain has to figure out that "cat" means a furry animal, "sat" means it's sitting, "on" means it's on top of, and "mat" means a soft thing to sit on.
Once your brain has figured out what all the words mean and how they fit together, it can understand the whole sentence and its meaning. This happens really quickly, so you don't even realize your brain is doing it!
Sometimes, though, sentences can be really complicated or have words in them that you don't know. When that happens, your brain might have to work a little harder to understand what the sentence means. But with practice, you can get better at sentence processing and understand even really tricky sentences!