Any permanents, spells, or creatures played on the previous turn are "unplayed." Anything that happened during the previous turn is undone, except for quanta and HP. It's like a Mass RT, except it works on Permanents and Spells, too, if they were played on the previous turn.
Example:
Player 1 has 12 Fire Quanta, draws and plays a Phoenix, and plays a Lava Golem from his hand. He ends his turn with 3 Fire quanta. Player 2 has 93 HP.
Player 2 has 7 Time Quanta and plays Adynata. The Phoenix is returned to the top of Player 1's deck and the Golem is returned to Player 1's hand. Player 2 ends his turn with 0 Time Quanta and 93 HP.
Player 1 has 3 Fire Quanta and is unable to play either the Golem or the Phoenix.