Using horn signals is a way to notify other boaters of what you're doing as you approach them (or vice versa), says the BoatUS Foundation. Here are some of the most common signals and what they mean:

One short blast: When crossing paths near another boater, this signal means you plan to pass their boat, leaving them on your port (left) side. If you are behind another boat, and about to pass it, one quick horn blast signifies that you will pass it on their starboard (right) side.
Two short blasts: This horn alert is the opposite of one short blast. It means if you're crossing paths near another boater, you will leave them on your right side. Or, if you are behind another boater, it means that you'll pass them on their left side.
Three short blasts: Use this horn signal to let other boaters know that you're backing up.
Five or more short blasts: A series of five short blasts is a danger signal. The BoatUS Foundation states that when another boat responds with the same signal, it means both boaters understand the intended maneuver. So, if you feel that another boat's horn signal is unclear or unsafe, use this signal to alert them of your uncertainty.
One prolonged blast: This alerts other boaters that you're leaving a dock. Some boaters also use it when they're headed around a curve and cannot easily see other boats coming.