The parachute that you deploy if your main chute gets messed up and you have to cut it while skydiving. In civilian rigs, the reserve is packed in the compartment above the main chute, and usually has a slightly faster deploy time than the main. Reserves are pulled after you have cut your main - if you deploy a reserve while still attached to your main, there is a good chance the two chutes will tangle as they vie for dominance above your head.
You had damn well better hope that your reserve works right or you are screwed, game over, that's all she wrote, the end of the line, see you on the other side, tell God I said 'hi' for me, will you?