Sunspots
Sunspots are dark patches on the Sun that look darker because they are cooler than the area around them.
Think of it this way
A pan of soup sits on the stove, hot all across the surface. Some spots near the edge cool down faster and look darker than the rest. Those dark, cooler patches on the soup work just like sunspots on the Sun. The Sun is hot all over, but sunspots are cooler areas — so they look darker against the bright glow around them.
Explaining sunspots by grade level
The Sun has dark spots on it. These spots are cooler than the rest of the Sun. You can see them by shining the Sun's image on paper. They look like small shadows on a bright circle.