A K'in is a part of the ancient Maya Long Count Calendar system which corresponds to one day

Day

A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

. As such, it is the smallest unit of Maya time to be counted as part of the long count and it usually appears as the last glyph

Glyph

A glyph is an element of writing: an individual mark on a written medium that contributes to the meaning of what is written. A glyph is made up of one or more graphemes....

 in a long count date. Such long count dates can be seen on many inscriptions in the Mayan area at the start of the initial series which usually occurs at the beginning of an inscription.

k'in means sun in Mayan language