Culture is a set of things that people do, say, and believe in. It can be different from one place to another and it can change over time.
Imagine you visit a new place with your family. They speak a different language, eat different foods, and have different customs than what you are used to. This is because they come from a different culture.
Now, imagine you are reading a book and you see a message that says “please turn to page 3” but you can’t find the page. That’s because the page number is different depending on the book you’re reading. Just like in culture, things can be different depending on the group of people you are with.
So, page 3 culture is talking specifically about the things that a certain group of people do, say, and believe in. Just like different books have different page numbers, different cultures have different practices, languages, beliefs, and traditions.