Impediments To A Valid Marriage:
There are numerous impediments to a sacramental marriage. An impediment is like a wall or barrier that prevents a sacramental marriage from taking place. It can be spiritually lifted in some cases. This waiver or lifting is referred to as a dispensation. It must be done by a bishop'. For example, lack of baptism is an impediment; it is dispensable. Being too closely related by blood would be an impediment that is not dispensable. As a bishop, I grant dispensation for all of the dispensable impediments immediately upon request.
See: Roman Canons 87, 391, 1078 and 1086