Lightmass is the high-quality static Global Illumination solver created by Epic games. Global Illumination (GI) means the process that simulates indirect lighting (for example, light bouncing and color bleeding from surfaces). In Unreal Engine, light bounces by default with Lightmass and is based on the base color of your material, which controls how much light should bounce from the surface of the object. Even though a more highly saturated color will bounce more light, and a less saturated color will bounce less, it all depends on the scene. In a simple room-like scene, this can be noticeable, whereas if it's an outdoor day scene this might not be that noticeable.

Let's take a quick look at the following scene:

This is a simple scene in unlit mode.

Now I added one Directional Light and this is how it looks with no GI. That means there is only direct lighting and no indirect lighting (meaning there is no light bouncing).

The previous screenshot is with static GI...