Something manual is operated by a human. Before Eli Whitney invented the cotton gin, picking cotton was a manual job. Now, farmers will need to read a cotton gin manual to know how to operate it. Good old Eli.
The word manual comes from Latin root words meaning "of the hand," and the adjective form of the word still maintains that meaning — "by hand or of the hands." The adjective definition is also extended a bit to mean something done by hand and not machine, and the implication is usually that it is physical work. As a noun, manual means "an instruction booklet or handbook."
