Superweapon

Depending on the game, a super weapon can be a couple of different things. 
 A superweapon can be used to drive a story for example. Since superweapons are known for beeing very very very powerful, the vilain in a story usually tries to gain control of it and use it for his evil deeds. A super weapon can be a robot, a bomb, a space station, a satellite, a gun, etc. Sometimes superweapons must be built by using other weapolns and sometimes the hero may be looking to use the superweapon against his ennemies (example: Gears of War). 
In other games, a superweapon may be awarded to the player for skill. This can happen in both single player and multiplayer games. A game may give you a superweapon with limited ammo to defeat a specific ennemy, usually a boss. In multiplayer game, the player may be rewarded with a weapon that'll turn the tide of the battle in his favor (example: the tactical nuke in Modern Warfare 2).  
From a gameplay perspective, super weapons aren't usually playable. When they are though they usually feel heavy but powerful. These gameplay sections are usually easy romps through a level with very little chances of dying or getting hit. Sometimes a game will pit two super weapons against each other which usually means a drastic change in the gameplay and controls (For example: Metal Gear Solid 4)
The most important thing to remember about superweapons is that whoever has one has a significant edge over their adversaries.