Easy definition:
It is a set series of prayers, psalms, and scripture readings that nuns, monks, priests, and other religious types are required to pray throughout the day at different hours, (every 3 or so, as in 3am, 6am, 9am, noon, etc.). They are different every day, and as the seasons change.
Some lay people also pray them as a spiritual exercise.
It is also called the Divine Office
They come in large books, or cellphone apps.