If a group of people are "on the same page", it means that they all have the same idea about what's going on. For example, if a group of people are working on a project together and they all know who is in charge of each part of it, then they are "on the same page". If some people think that Mr. A is in charge of something, and other people think that Mr. B is handling it, then they're not "on the same page".
Here are some more examples of how "on the same page" is used:
Just to make sure we're on the same page, you know that this position is unpaid, right?
I think we should all get together and talk, just to make sure we're on the same page.
Yes, I agree. We're definitely on the same page on that.
Here's a good way to remember this phrase: imagine a classroom full of students discussing something in a textbook. If all the students are on the same page in the textbook, they can talk about it productively. But if some of the students are looking at a different page in the book, they'll be confused.