In plain words
A helper in the service, often a young person, who lights the candles and carries the cross.
The fuller definition
A helper in worship, often a young person, who lights and puts out the candles, carries the cross or the processional torches, and assists at the altar. From the Greek for “follower” or “attendant.” In many congregations it is the first job a child is given in the service.
In Sunday worship: The parts of a service, daily prayer, and the things you see in a church.