The frenum is the muscle tissue that attaches the upper or lower lips to the gum. Another frenum attaches the tongue to the floor of the mouth. When someone is tongue tied the frenum that attaches the tongue to the floor of the mouth is too thick and attaches to gum tissue in the area behind the teeth instead of being lower in the floor of the mouth. So the tongue can protrude properly, a frenectomy is needed to sever the frenum.
The frenum can interfere with tooth alignment or can cause gum recession by pulling on the tissue in front of the tooth. These are two other indications in which the frenum needs to be severed.