When the sun is lighting up the sky outside, it's daytime. Most adults are at work during the daytime and come home at nighttime.
While daytime is defined as "after sunrise and before sunset," it describes this time of day even on dark, gray, days, or in parts of the world that get limited sunlight during the winter. A more accurate way to think of daytime is the hours after you wake up in the morning and before you start to think about going to bed at night. When it's daytime where you are, it's nighttime on the opposite hemisphere of the Earth.