Pretty much it is what sets are currently allowed.
A rotation in the Pokémon Trading Card Game is when several sets are removed from Play! Pokémon sanctioned events, generally because they are too old and would not be compatible with newer sets or rules. Rotations almost always occur directly following the World Championships and remove four or more sets. If a set is rotated, that means a player may no longer use any card from that set in their deck (with exceptions; see below). This rule only applies to Standard format (formally referred to as Modified format) and Expanded format. In Unlimited format, players may use any card from any set in their decks.
For more information, reference Bulbapedia's rotation article.
