what does " to the extent" mean ?

You need to provide context. If we don't know what the sentence is in which you read it, we can't give you a truly intelligent answer.

However, "to the extent that" means a condition of limiting--sometimes, but not always. So, i might write: "To the extent that he knew it was wrong, he was guilty." That means that someone did something wrong, and because he knew it was wrong, he was guilty--in this case, using to the extent that shows that there was no limiting factor. But i might also write: "She understood the answer, to the extent that she understood the scientific terms used." That means she could only understand the portions of the answer which involved scientific terms which she already knew, but didn't understand the portion of the answer which used terms which were unfamiliar to her.

Context, we need context to give you intelligent answers.