What do red and green lights on a boat mean?

The red and green lights on a boat are side lights and assist in determining the direction the boat is facing. Red lights are on the port, or left side, while green lights are on the starboard, or right side. If you see only one side light, you are looking at that side of the boat. If you see both lights at the same time, that means that the boat is either headed towards you or away from you. If the red light is to the right, the boat is coming towards you (Red-Right-Returning). If the green light is to the right, the boat is going away from you.

See the associated link for US Coast Guard Navigational Rules of the Road