A sacrament is a religious ceremony or rite. Sacraments are important holy events to members of that religion.
Different religions recognize different sacraments. If you're a Protestant, you might attend a baptism, yet if you're Roman Catholic, you would include confirmation and matrimony among the various ceremonies you attend. Usually a holy person like a priest or minister needs to perform a sacrament. A less religious definition of this word is to think of a sacrament as something that is sacred or mysterious: a serious baseball fan might think of visiting Fenway Park as a sacrament.