A sunspot is a dark patch on the surface of the sun. It looks dark only by comparison — it is still fiercely hot, just cooler than everything around it, because the sun's magnetic field is twisted up at that point and slows the heat rising from below. Sunspots come and go, and their number rises and falls in a cycle of about eleven years; when there are many, the sun throws out more storms of particles, which can disturb radio signals and satellites and give us bright auroras near the poles. Astronomers count them, so you will hear 'sunspot number' and 'sunspot cycle' in science writing. It is a countable noun: one sunspot, many sunspots. In holiday advertisements you may also see 'sunspot' used loosely to mean a sunny place to go on holiday, and doctors sometimes call the small brown marks that come from years of sunshine sunspots too — but the astronomy meaning is the main one, and the one being explained here.