It’s the distance from the lens to the focal point, as in where the picture focuses on the sensor behind the lens. If you have a very long focal length like a telescope, you can see things further away but the range you can see is very small. With a short focal length you can’t see as far but you can see a much wider view. Check out this chart:

Image

If you get very close to something with a short focal length or far away from it with a long focal length you can get essentially the same picture of a main subject (although what you can see in the background will be different), but even then a short lens will sort of taper your subject closer to a single point and a long lens will widen it. You can see this effect easily on faces: see this gif or this gif or this picture for an example.