The Horizon is that imaginary line on the Earth's surface that marks the furthest point that can be seen from your vantage point. The exact distance to the horizon depends on your altitude, and increases as you get higher. That's why the lookout on a ship was always posted at the highest point on the ship.

The exact values depend, or course, on the local topography, but we usually get close enough simply by assuming the Earth is a sphere. We need to know the size of the Earth to do the sums.

Interestingly, the distance to the horizon calculation is effectively identical to the calculation of orbital velocity.