Erratas are changes to any part of the text of a card (other than Card Number) that occur between reprints.
Why there are Erratas
Erratas can be made to a card for a variety of reasons, including to fix mistakes, to update a card's terminology, or to clarify the behavior of a card so that its rulings are easier to understand, such as Problem-Solving Card text. Most Erratas change the Card text in order to make the card less powerful so it can be removed from the Forbidden list.
Most Recent Errata
You are not required to own the most recent erratum of a Card, but they must always be played according to the latest erratum even if you are using an older copy of the Card. Konami recommends using the most recent print of a Card, because that text is the most accurate.