This light can be used to simulate sunlight coming through a window
This is a parallel light from a distant source, specified in terms of a direction vector. Often used to model sunlight.
Directional lights in Maya are often called 'distant' lights in other software.
The Lights page has more detail about the controls. In addition to honoring the standard Maya light attributes, the Attribute Editor will also show the following attributes under the Arnold group:
The angular size of the light, in degrees. Non-zero values produce realistic soft shadows. For example, the Sun subtends approximately 0.5 degrees as seen from the surface of the Earth. A setting of 1 or 2 will produce slightly softened shadows like hazy sunshine, larger numbers such as six will be much softer.
Note that increasing the angular area of the light will make it brighter, unless 'normalize' is on.