An Insert Game, or Insert War, is the general term for a type of game in which players insert user made models in order to fight each other with. Insert games usually consist of an insert tool within the player's inventory and a simple baseplate on which to do battle on. Games of this genre are generally criticized for the ease that one can do to abuse and spam within the game.
Before the InsertService update released, insert games were commonly abused. Players had the ability to insert game breaking models, such as a forever exploding bomb and admin scripts (which players could then use to loopkill players, kick/ban players, or even prevent new players from entering). Also, they could just insert nukes to delete evething within the game, which is still possible today, even if the brick is locked. To add, prior to the InsertService update, certain models called "takeovers" manipulated the game to only allow the model owner to control the game, preventing others from using the main tool.
A recent update to the InsertService service has ultimately destoyed insert games as InsertService now only permits the insertion of official ROBLOX assets (such as hats) and models that the creator of the game owns. The pros and cons of this update varies: insert games are less likely to be abused, but are limited to using primarily gear as a result.Now, you can only insert bricks, clothes and hats.