"Drop in, drop out" usually refers to online video game playing where any player wishing to join the game in progress can "drop in" and assume instant control of a NPC
(computer controlled player), without any significant disruption to the gameplay. The player can also leave and return control of the character to the computer - "drop out" again without causing disruption to the game.
Gears of War
offers drop in, drop out gameplay where any player can assume control of Dominic Santiago, Marcus Fenix's best friend, without freezing or restarting the level. World of Warcraft
also offers this feature.