What does red green and white lights on a boat mean at night?

At night, boats are supposed to display navigational lights; these are red, green, and white. The lights are screened/shielded so that each light is only visible from a certain range of directions.

If you are on a boat and can see the red, green, and white navigational lights from another boat all at the same time, you should watch out, because that means that it is moving directly toward you and there is the possibility of a collision.