New Game Plus (also written New Game +) is a recurring feature of the Chrono series: when the game is completed you may play through again keeping everything you had from the first playthrough.
With the New Game + feature, one is able to keep all of their items, their status, etc. It also allows the player to even access certain endings that are not able to be received the first playthrough.
In Chrono Trigger, most endings are achievable without a New Game Plus game save, it is easier to view all endings this way. An ending exclusive to New Game+ is The Frog Ending. To get this ending, fight Lavos through the right Telepod after rescuing Queen Leene from Manolia Cathedral. Contrary to popular belief, the developer room ending is accessable with out new game +. In order to achieve this one must beat lavos in 12,000 BC when Crono is supposed to die.
In Chrono Cross, the New Game + feature allows far more than just a single ending. It grants many different new items such as starting off with the Relief Charm and the Time Egg granting access to fighting the Time Devourer at any point in the game which will grant the ability to view the other endings by defeating the Time Devourer at different points in the game. Also with a New Game+ by holding L2 you can slow down the game or speed it up with R2. New bosses become available in the Bend of Time: Ozzie, Flea, and Slash. You lose your Key Items, including the Chrono Cross and the Mastermune. Lastly, it is possible to use the Chrono Cross once you get it back to get all of your characters from your previous game file including Harle but excluding Lynx.