In Triplets, each side starts with a pawn move. In the second turn, each side plays a double move consisting of a pawn move and a move of a non-king, non-pawn (a 'piece'), in either order. Each subsequent turn is a triple move (hence the name) consisting of a pawn move, a king move, and a piece move, in any order.
 
A player loses when checkmated or unable to complete any part of his/her move. Most games end when one player runs out of pawn moves, so keep an eye on your pawns!
 
For more information, see the Triplets page at The Chess Variants Pages.