sunspot
A sunspot is a dark blemish on the bright face of the Sun, a patch of the surface that looks black or grey against its surroundings. They come and go over days and weeks, and they have been watched and sketched by astronomers for centuries — they were among the very first hints that the Sun is not a perfect, unchanging orb.
A sunspot is dark only by comparison: it is cooler than the surrounding photosphere, perhaps 3,500 to 4,500 degrees Celsius instead of about 5,500, and cooler gas glows less brightly. The cooling happens because an intense knot of magnetic field, thousands of times stronger than Earth's, pokes up through the surface there and chokes off the normal upward churn of hot gas. A large sunspot can be many times the size of the Earth, and most have a dark central region (the umbra) surrounded by a lighter, feathery ring (the penumbra).
Sunspots matter because they are the most visible sign of the Sun's magnetism and the clearest marker of the solar cycle. Counting them over time reveals the roughly 11-year rise and fall of solar activity, and the regions around big sunspot groups are exactly where flares and coronal mass ejections tend to erupt, making them important for space-weather forecasting.
If you could lift a sunspot out and place it on its own in the night sky, it would not be black at all — it would shine brighter than a full moon. It only looks dark beside the even brighter Sun.
Sunspots are dark only by contrast, not in any absolute sense.
Sunspots are not holes or cracks in the Sun. They are cooler magnetic patches on the same gaseous surface, and the gas flows around and through them.