A roof truss is a frame, typically made out of wood. The frame is composed of a series of connected triangles as that particular shape has great structural stability.
A roof truss does a couple of different things. Primarily, it supports the components of a home's roof, including roof sheathing, felt paper and shingles. Secondarily, a roof truss adds structural integrity to the home's envelope by helping tie it together from the top.
Without roof trusses a building would have no platform for a roof. Further, the building envelope would be significantly weaker.