In "Roller Skate Man," Raymond Souster describes a disabled man who transports himself on a block of wood that sits on top of roller-skate wheels; the man propels this gadget by pushing his hands onto the hard pavement.
The central metaphor of the poem compares this man and his vehicle to a boat traveling in water:
Steering through the familiar waters
Heavy with spit, old butts, chewed gum
Flotsam among the jetsam of this world.
Flotsam and jetsam are various kinds of debris from ships that are found at sea.
More specifically, flotsam (from the word float) refers to items that are floating because of the natural action of the sea. For example, pieces of wood from a shipwreck would be called flotsam.
Jetsam (from the word jettison, to abandon) refers to items that have been thrown overboard by a ship's crew. Food scraps, broken tools, (even men thrown overboard as punishment!) would be considered jetsam.
Souster adheres to this technical distinction very carefully. He refers to "spit, old butts, chewed gum" as jetsam, because these are things that have purposefully thrown to the ground. The Roller-Skate man, however, is referred to as flotsam. No-one has deliberately cast him to the pavement; rather, he has landed there as a natural consequence of the stormy waters of life.