Something manual is operated by human hands. Before Eli Whitney invented the cotton gin, picking cotton was a manual job. Nowadays, farmers might need to read a manual to know how to operate a cotton gin.
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."