During a eclipse like the one happening today, the moon passes infront of the sun during daylight hours. As it passes across the sun, for a section of the Earth’s surface the sun’s light is blocked out, either totally (A Total Eclipse) or some percentage of the sun’s surface (a Partial Eclipse).
In the UK we are experiencing a partial eclipse today, with between 97% and 84% of the Sun covered. The further north you are the better today!
Be careful though, the sun is always dangerous to look at with your eyes, and during an eclipse while it seems safe, it is still dangerous to your eyes (not more dangerous, just the same) a bit like how you can get sunburned on a cloudy day. If the clouds clear where you are then use special goggles or pinhole camera’s to see the eclipse.
James is right, it’s the moon passing between earth and the sun. The moon is very small, and the sun is very large, but because the sun is so far away (about 150,000,000 km) it looks much smaller and the moon is close (only around 400,000km) so it looks larger in comparison. It’s actually unique as we’re the only planet in the solar system that the distances and sizes are just right to cause this wonderful effect