It is a kind of psychotherapy, also known as couples therapy. The counselor helps couples to resolve their conflicts and start their relationship freshly. In easy words we could also say, marriage counseling helps couples with all types of relationships regardless of marital status and sexual orientation.
Marriage counseling is mainly required by the couples who want to strengthen their relationship and to begin better understanding between each other. Couples can also go for premarital counseling to iron out differences before marriage. Issues could be infidelity, anger, communication problems, sexual difficulties, etc.