In videogaming, this means exploiting a bug in the code in order to cheat or do something that you're not supposed to be able to do (travel underground, etc). For more, see wikipedia:Glitching

one glitch that can come very in handy is the grenade glitch say your on a cliff and you jump of looking down when your getting closer to the ground quickly press the jump button and the L1 button at the same time and you will survive the landing P.S. make sure you have grenades.