A comb is a part of harp usually made of wood, plastic or aluminium - that holds the reed plates. The comb, as the name suggests, have numerous rectangular partitions at both side whose top side is to be covered by a reed plate and thus form a blocked area having two walls and covered by the reed plate. By this design, it seperates on or more reed(s) attached to the reed plate into seperate room so that when you blow/draw air, it passes only through the room and thus a single/set of reeds are affected.
While the comb is the matrix of the harp, the material of the comb plays very little role in tone quality. However, the duration of the harp is dependent on the comb and the wooden comb is said to suffer from the saliva in the players breath. Wooden comb often swells or cracked by aging and hence the plastic comb or more costly aluminium combs are more and more in use.
It is imagined by some folks that wooden combs play more mellow tones, but without solid proof.