Final answer:
At night, the red sidelight indicates the right side of the vessel.
Step-by-step explanation:
At night, the red sidelight indicates the right side of the vessel. This is important for other vessels on the water to determine which direction the boat is moving and to avoid collisions. The red sidelight is also known as the port light. On the left side of the vessel, there is a green sidelight which is known as the starboard light.