Okay kiddo, have you ever seen a really fancy piece of fabric with special designs on it like what a king or queen would wear? That's called a "serge".
In some religious traditions, they use a special type of serge cloth for things like covering the altar, table or other important items during their religious ceremonies. This special type of serge can symbolize purity, holiness or sacredness - which are important ideas in many religions.
Some people may even wear special religious garments made out of serge cloth, which act as a reminder of their faith or religious beliefs. So basically, serge cloth is just a special type of fabric that is used in some religious traditions for important ceremonies and clothing.