Meaning that only cards from the sets Black&Whitee to Boundaries Crossed are allowed to be used.
The sets allowed are:
Also allowed are any Promo cards released during this time, as well as the mini-set Dragon Vault. Any future sets will also be allowed, such as the upcoming Plasma Gale set. However, it is not known if the introduction of new sets will lead to new bans.
The reason for this is to make the game more balanced. Despite the power creep, there are plenty of cards from the past who would be too powerful for now (such as Jungle Mr. Mime), especially Trainer cards like Bill and Professor Oak now that game mechanics have changed.