When people write things like books, they own the words they put together. It's like owning a toy that you made out of blocks. You made it and it's yours.
Sometimes, people let other people read their books or use their words. But just because other people read it, it doesn't mean they own it too.
When someone writes something down officially, like in a book or a website, they have something called a "copyright". Copyright means they own their words and nobody else can use them unless the owner says it's okay.
So just like you wouldn't want someone to take your special toy without asking, you don't want someone else to take your words without asking either.