What is the function of a valve tendon in the heart?

The valves are attached to the heart wall by tendons as this allows the closure of the vavles to be controlled it also makes sure the the valve is completely closed to prevent backflow, which is potentially fatal.

OK i can only assume the question is reffering to the tendinae chordinae which connect the atrioventricular valves to the pappilary muscles projecting from the heart wall? The functions of these structures is to prevent the massive pressure difference, between the vetrivles and actria during systole, from pushing the the valve inside out and thus preventing backflow. NB the pappilary muscles and chordea tendinae are not involved in clossing or openinghte valves they just stop inversion.