Light behaves like a flow of particles,  called photons, which travel at the speed of light. Photons are emitted by primary sources (the Sun, light bulb filaments, etc.) and can be reflected or absorbed by other objects.
The Moon is a secondary source of light because it only reflects a part of the light it receives, without emitting any.
An ideal mirror would produce optimal reflection of light, as it would reflect all the photons that strike it.