Crepuscule is twilight or dusk. It’s that period of not quite dark and not quite light, right after sunset.
Night is dark, while day is light (as long as it’s not cloudy). Then there are times when it’s mixed: like crepuscule. Crepuscule is a time of partial darkness, as the sun has just set. This in-between time is also called dusk. In fact, English is blessed with many words that mean dusk — such as evenfall, gloam, gloaming, and nightfall — but when you want to set a poetic tone, crepuscule (which comes from Latin) is hard to beat.