Subsistence means the minimal resources that are necessary for survival. If you work for subsistence, you'll probably receive food, water, and lodging (internet access not included).
Subsistence can be used in a variety of subtly different ways. In addition to referring to necessities for survival, it can indicate one's means of obtaining those necessities (usually a job). And if you feel like getting philosophical, subsistence can refer to the very act of existence itself.