Tendons are vertical mooring lines of a floating platform. A Tension Leg Platform’s Tension Legs are comprised of one or more Tendons per Leg. An example of that would be a conventional column hull, 4 Leg configuration, with 2 Tendons per Leg.
The Tendons hold the platform deeper than needed just to float. The excess buoyancy puts tension in the tendons and keeps the platform from moving around as much.
The Tendons also hold the platform level as it offsets, limit yaw and prevent pitch, roll and heave.