Hungarian grammar is like a set of rules. Just like when you play a game, there are rules you follow so everyone knows how to play. In Hungarian, there are rules you have to follow so that people can understand what you are saying.
One rule is that the order of the words can be different than what you might be used to. In English, we usually say "I eat an apple." But in Hungarian, you can say "Apple eat I." It doesn't sound exactly the same, but it means the same thing.
Another rule is that words can change depending on how they are used. For example, if you say "the apple is green" in Hungarian, you have to change the word for "apple" so it matches with "the." In English, we don't change "apple" at all. But in Hungarian, it's important to match the words correctly so people understand you.
Hungarian grammar has 18 different cases, which is a way of showing how words are used in different situations. Each case has its own set of rules, so it can be tricky to learn all of them. But just like with any game, the more you practice, the better you get.