Final answer:
The red sidelight on a vessel indicates the port (left) side of the vessel at night.
Step-by-step explanation:
The red sidelight on a vessel indicates the port (left) side of the vessel at night. This is in accordance with the International Regulations for Preventing Collisions at Sea (COLREGs). The red sidelight helps other vessels determine the direction in which the vessel is moving and avoid any potential collisions.
For example, if you see a red sidelight on the left side of a vessel, you should give way to that vessel since it is on your starboard (right) side.