Sunlight

Sunlight, in game, is the name given to the light which illuminates all blocks which do not have shadow cast over them by a non-transparent block positioned vertically above them in the sky.

Function
Sunlight is one of the two requisites to turn dirt blocks into grass blocks (the other being that there must be another sun-affected grass block adjacent to it). Sunlight alters the brightness of the blocks it falls upon. Patterns suspended in the air for sunlight to shine through affect the colours on the ground. This can be used to provide more shades of colour for more compliacted patterns on floors and such.