A grammar checker is something that helps you check if your writing is correct. It's like having a teacher or a smart friend look over your work and tell you if you made any mistakes.
When you use a grammar checker, it looks at things like your sentences, your spelling, and your punctuation. If it sees something that's not quite right, it might show you a little red underline or give you a suggestion on how to fix it.
Just like when you are learning to read and write in school, using a grammar checker can help you get better at writing and help you catch mistakes you might have missed on your own. But it's important to remember that a grammar checker is not perfect and sometimes it might miss something or suggest something that isn't quite right. So it's always good to have a real person, like a teacher or a parent, look over your work too.