Serbo-Croatian grammar is like the set of rules that tells us how to put words together in a sentence so that they make sense.
Imagine you have a bunch of legos and you want to build a house. The rules for building the house would be like the grammar rules for Serbo-Croatian. Just like you need to put the lego pieces together in a certain way to make a house, you need to put the words together in a certain way to make a sentence in Serbo-Croatian.
In Serbo-Croatian, the order of words is really important. For example, if you were saying "I love you" in English, you could say "You love I" and people would still understand what you mean. But in Serbo-Croatian, you have to say it in the right order: "Volim te."
There are also different endings for nouns and verbs depending on how they're being used in the sentence. For example, in English we would say "I have a book" or "She has a book." But in Serbo-Croatian, the word for "book" would change depending on whether it's the subject (like "I" or "she") or the object (like "a book").
Basically, Serbo-Croatian grammar is like a big set of instructions for how to put words together in a sentence so that people can understand what you mean. It's important to follow the rules, just like it's important to follow the instructions when you're building something out of legos.