This is the 306th most frequent Russian word.
днем
"During the day" or "in the afternoon."
'Днем' in this usage refers to the period suitable for daily activities that happen in daylight hours.
Днем я обычно занимаюсь своими делами.
During the day, I usually take care of my tasks.
In this sentence, 'днем' describes the time when the sun shines—indicating the daytime hours.
Солнце светит особенно ярко днем.
The sun shines especially brightly during the day.
Here, 'днем' is used to specify the time of the action, meaning 'in the daytime' in English.