New Game Plus is a gameplay feature of Chrono Trigger. Once players have defeated the boss Lavos in a normal playthrough, they can begin a new game with all of their previous equipment, levels and Techs.

The boss Lavos is encountered multiple times throughout the game. Chrono Trigger's ending changes depending on when the player defeats Lavos in the game. Taking advantage of the New Game Plus feature, players can beat Lavos earlier in the game, at a time when they would otherwise be too weak, and thus access "The Dream Project" game ending.GamesRadar: Chrono Trigger Game Endings

New Game Plus is not a new gameplay concept. The term was first used in the original Chrono Trigger game. However, two of the first games attributed with offering a New Game Plus element are Super Mario Bros. in 1985 and The Legend of Zelda in 1987. Since then, hundreds of games have offered variations of the New Game Plus gameplay element, although it might be called something else and the specifics of the mode differ from game to game. In addition to allowing players to retain items, spells or experience, some games also alter the level layout or make enemies more difficult to defeat.GiantBomb: New Game Plus Wikipedia: New Game Plus