Refers to the middle of the day when the sun is up; often used in contexts emphasizing that something happens during daylight hours.
It's still hot during the daytime, so I'll take a walk in the evening.
犯人は日中に堂々と侵入した。
The culprit brazenly broke in in broad daylight.
A native compound of 日 “day, sunlight” and 中 “middle,” meaning the midst of the day or broad daylight.