Okay, so you know when you finish reading a book and you tell your friends or family about it and say whether you liked it or not? That's kinda like a book review!
But, sometimes people write down their thoughts about a book and put it online, in a magazine or newspaper, or even in a book itself! This is called a book review too.
A book review is basically someone's opinion or thoughts on a book. They might say if they liked it or not, what they thought of the characters, the story, the writing style, or anything else they noticed.
It's helpful to read reviews if you're trying to decide if you want to read a book or not. You can see what other people thought and decide if it sounds like something you would enjoy.
Sometimes reviews can be really detailed and long, and other times they're short and simple. But either way, they're just someone's thoughts on a book and can help you make a decision on what to read next!