Subsistence farming is the kind of farming done by farmers who have small plots, enough only for themselves. Literally, subsistence agriculture means no extra food is produced to sell or trade. This means farming doesn't give them money to buy things. However, today most subsistence farmers also do trade to some degree. From time to time they may need money to buy essential things to keep going.
Subsistence farming may also mean shifting farming or nomadic herding (see nomadic people).
Examples: