garden roller vs roller

garden roller and roller both are nouns.

Word	Noun	Adjective	Verb	Adverb
garden roller	Yes	No	No	No
roller	Yes	No	No	No

As nouns, roller is a hypernym of garden roller; that is, roller is a word with a broader meaning than garden roller:

garden roller (noun)	roller (noun)
heavy cast-iron cylinder used to flatten lawns	a grounder that rolls along the infield
	Old World bird that tumbles or rolls in flight; related to kingfishers
	pigeon that executes backward somersaults in flight or on the ground
	a cylinder that revolves
	a mechanical device consisting of a cylindrical tube around which the hair is wound to curl it
	a small wheel without spokes (as on a roller skate)
	a long heavy sea wave as it advances towards the shore