Lateral marks are used to define the edge of a navigable channel, most commonly when entering/leaving harbours, and consist of 2 different buoys, a port lateral marker and a starboard lateral marker. Red is used for one side of the channel and green for the other side, depending on which IALA buoyage system you are currently in depends on which colour is used for which side. Also which side of the channel the respective buoys are placed depends upon the direction of buoyage.
IALA Buoyage System A
View IALA A Lateral Marks in more detail
IALA Buoyage System B
View IALA B Lateral Marks in more detail