Frag Grenades are explosive weapons carried by a certain number of Unit Types. When on hand, the grenades could be held to be thrown a longer distance. When reaching a point, they will explode, making a large blast radius. Whoever is in the radius will get damaged, though ones closer into the blast will get hurt more.
The Frag Grenade is one of the most useful tools in the game. It is used by many pros to first weaken an enemy, and then to kill them off. People who don't use Frag Grenades are at a huge disadvantage, against every class.