Compline is a liturgical service, monastic in origin, which is celebrated in the late evening. In the Byzantine Rite, it has two different forms: Small Compline, which is celebrated on most days, and the Great Compline, a longer and more solemn service which is celebrated on the eves of the Nativity of our Lord, Theophany and the Annunciation, and on certain weekday evenings of the Great Fast.
Great Compline presents us with themes of gratitude, repentance and praise, while small Compline revolves around the Canon appointed for the day, and the Prayer of the Hours.
Depending on local tradition and circumstances, the time of Compline may vary widely. Compline is not normally combined with other services, although it may be followed immediately by Matins on the eve of the Nativity, Theophany, and the Annunciation.
Compline can be celebrated in the whole or in part, as evening prayer at home. The fixed order and the service of mutual forgiveness at the end make it appropriate as a family prayer before bedtime.
Adapted from: https://mci.archpitt.org/liturgy/Compline.html