Imagine that you are on a ship sailing with some pirates "Arrrr!!". You are looking out over the water. You think to yourself as you watch the shimmering on the surface, "Just how far away IS the horizon?" This calculator with tell you how far it is.
A bit later you see your friend on another ship pass you by. He's in the crow's nest. You watch as he sails towards the horizon. You watch the ship dip below the horizon, but you can still see your friend in the crow's nest. This calculator will tell you how far your friend can go before he dips below the horizon.
Just enter how high your eyes are from the surface of the water and how high above the water your friend is. If you want to know how far the horizon is, just enter 0 for target height.