Not exactly. After a book is officially published, they inevitably discover that there were some mistakes made in that book: typos that said the wrong thing or places where an additional explanatory sentence was really necessary to make the rules work the way they want them to work.
It's not new rules, just clarification to the published rules in the books- and usually just a couple pages of clarification is necessary.
They've got an errata for most WOTC books, and they are free, so go ahead and download a couple and take a look: