What Does "Praying the Office" Mean?

I love Father Tim of the Mitford books written by Jan Karon. And in one of the books, the author wrote about him praying the office. I didn't know what that meant.

I also loved our pastor at the time, Dr. C. And he would mention in sermons how he followed the Lectionary. I didn't know what that meant either.

But I was intrigued.

I hope to write a small series about what I learned.