Three hours? Yikes.
I usually just explain that religious choose celibacy and dedicate their lives to serving others. Some work in hospitals, homeless shelters, crisis centers, schools, orphanages, etc. They all work to help mend our society. Most people are generally pretty taken back that someone would give up a personal career, marriage, and biological kids for such a simple life.
At the root of it, all religious serve others (as all vocations should, whether vowed religious or not). Try just explaining it as a life of service.
The point of it? To help and love other people.
And in honor of the recent release of Les Mis…“To love another person is to see the face of God”.