Originally, the purpose of the chrisom-cloth was to keep the chrism, a consecrated oil, from accidentally rubbing off. With time, the word's meaning changed, to that of a white mantle thrown over the whole infant at the time of baptism.
Initially, the christening cloth served as a protective barrier for the consecrated oil, but its significance shifted to represent a white garment enveloping the infant during the sacrament of baptism.