A five-spot is a five-dollar bill — paper money worth five dollars. It is informal American talk, a little old-fashioned now, the sort of line you hear in older films: someone slips you a five-spot. It is not a bank word, and you would not write it on a form. Though actually the same pattern shows up in ten-spot for ten dollars, so number + spot can mean that many dollars in this slang. Do not picture a spot of dirt, or the spots on an animal.