Power-driven vessel underway, under 50 m

Lights

one white masthead light

red and green sidelights

white sternlight

Viewing aspect

Ahead: masthead plus one or both sidelights depending angle.
Astern: sternlight.
Abeam: appropriate sidelight plus masthead light.

Means

Power-driven vessel underway.

Nuance

A second masthead light is optional for many vessels under 50 m.