A DC power supply is a device that supplies electric energy of fixed polarity, either positive or negative. "DC" in this context means "direct current." Direct current is an alternative to alternating current, frequently shortened to "AC."
Power supplies take energy from a power input and deliver it to a power output. The input may originally come from batteries, fuel cells, generators, alternators, solar power converters or something else. The power supply takes this energy and gives it to an output, referred to as an electrical load. A DC supply can be powered by either a DC source or an AC source.