It means the last actual turn, no matter who's turn it was then or is now, the transform ability checks on every player's upkeep, werewolves can actually transform back and forth every single turn.
It's more clear how it works in multiplayer games, when it's not just your turn then your opponent's turn. Lets look at an example:
It's your turn and you are playing one werewolf threat, since you don't want the rest to transform, say Huntmaster of the Fells, the blue player doesn't want you to do that, so they counter your spell. So far no single player has played two spells so your werewolves won't go back to human yet, but the blue player who's turn is next wants them to be human again and weaker, so he casts Brainstorm before your turn ends. At the start of the blue player's turn your werewolves all transform since a player cast two spells, but this is a blue player, and he has nothing but counters and land from the cards left in his hand, so he casts nothing, and no one else casts anything, he ends his turn and the next player's turn begins. Because no one cast anything, all your werewolves transform again back to wolf side, this player plays a spell and the blue player counters it. Next it's your turn again, two spells were cast but since each player only cast one, your werewolves stay wolves and you get to bring their wolf side strength into combat on this turn.