The character for sun was originally a picture of the sun. This is clear from the Oracle Bone Script and the later Bronze Inscription, which consist of a circle, which is a picture of the sun, with a dot inside. Modern characters do not include circle strokes, so the character has become stylized into straight lines, based on the strokes in modern use.
This character is pronounced rì.
This character refers to the sun. It also means day, the unit of time of the sun, since the sun crosses the sky in a daily cycle. 一日 means day 1, the first day of the month, and also 1 day, a length of time.