Yeah, as the other people already said with a short stack it's simply push or fold. With a short stack i mean 10BB or less, usually. So called "independent chip model" or "ICM". You have to push or fold, because mathematically seen, you have not enough chips for a postflop moves. Let's say you have a good hand and making standard x3BB raise. So at this point you are already pot committed, as you have left only 7BB in your stack and the pot is as minimum 4,5BB if you was first to act. This is already more than a half of your stack and means you have to fight for it further if hit or not. So why not just put more pressure at the opponents on preflop already, to gain extra fold
equity from them? That what it is about. I hope i could explain it clear.
Ah yeah, if you are new to this concept and not sure which range you usually push in ICM, i would recommend you to train yourself with a freeware software ICM Trainer. It gives a great help.