A carol is a religious song that's sung around Christmas time. In some towns, people who celebrate Christmas go to neighbors' houses and sing carols.
The most well-known carols are Christmas carols, but word applies to other religious songs as well. Various hymns and folk songs are sometimes called carols, and you can use the word as a verb to mean "sing a carol." In the 14th century, carol meant "to sing," but also "to dance in a ring," from the Old French carole, "ring dance accompanied by singers," probably from the Greek khoraules, "flute player who accompanies the dance."