Characters, vehicles, etc. are shot a great distance via a cannon, catapult, or other similar device as an unconventional means of transportation.
Alternate name: Unit cannon
The first video game about Human cannonball was released on July 30, 2009.
How this works depends often on the game genre, in most cases this is likely simply an "entrance" type device to one or more locations. In strategy games and similar this is a way to send your troops to a distant location fast.