Directional light
Light arriving in parallel rays from one direction, like sunlight, equally strong everywhere.
See it in Depth terrain. Try a sample map, then turn Sun angle and watch the light sweep across.
In plain words
The sun is so far away that its rays arrive side by side. A directional light copies that: every ray points the same way. It has a direction but no position, and it does not fade with distance. Every slope facing it is lit the same, wherever it is. It is the usual choice for daylight in 3D scenes.