This describes the orientation of another vessel from your perspective. It is a crucial concept in collision avoidance, especially at night as it determines what lights you should see.
It is defined as the relative bearing of your vessel as seen from the other vessel. It is measured from 0° to 180° and labeled Red when we are on the port side of the vessel or Green when we are on the starboard side. Thus a vessel with aspect Red 270 means you are looking at its port side. Green 45 or 45 green means you are looking broad onto its starboard bow.
See also Tricky Terms in Navigation.