A marriage spot decides where colonists will hold their marriage ceremony. During the ceremony, the married couple will stand on the spot. Other colonists will watch the couple from a distance. Attendance can also be arranged their location when placing dining chairs.
Drafting either side of the marriage will lead to the marriage being called off. If you happen to be playing with 5 colonies at once and they are not a day long apart from the ceremony taking place, it's possible to hold the event until everybody makes it to attendance by holding one of the two drafted outside of the room.
There is no cost, build time or skill requirements for placing marriage spots. Just select them from the menu and place them.