Bottom is a BDSM term for either a submissive partner in sexual play, or a partner who receives stimulation from another (the top), and who may or may not be submissive.
A bottom in BDSM does not have to be the receptive partner; for example, a dominant may command their submissive to penetrate them.
The main difference between a submissive and a bottom is that the submissive does not give instructions, although they do set limits on what the dominant can do. A bottom is not necessarily a submissive; they may enjoy intense physical and psychological stimulation but not submit to the person delivering it. Similarly, a submissive might not be a bottom; they may enjoy taking orders from a dominant without receiving any physical stimulation. For bottoms who are not submissive, the bottom is most often the partner who is giving instructions—the top typically tops when, and in the manner, requested by the bottom.