AT-43 Tactics is the roleplaying game system for use with the AT-43 miniatures wargame published by Rackham.
Unlike the tabletop miniatures game, AT-43 Tactics is intended to be played by small units of 3-6 player characters (PCs), while a "Game Leader" (GL) provides the impartial challenge of the missions this unit undertakes. As with many roleplaying systems, PCs gain experience points and levels from mission to mission.
Generally speaking, players take on specific roles within the unit: officer/leader, mechanic, medic, and so on.
Included in the AT-43 Tactics book are six missions comprising a short campaign. GLs are encouraged, of course, to create their own missions or run those currently available.
For further information on the world of AT-43 as well as the essential mechanics of combat, see the boardgamegeek entry for AT-43 and its many supplements.