A frenum is a naturally occurring muscle attachment, normally seen between the front teeth (either upper or lower). It connects the inner aspect of the lip with the gum. Sometimes a frenum is attached too high on the gums and can lead to recession or create spaces between teeth. The removal of this tissue attachment is called a frenectomy.
Some patients require frenectomies before/after orthodontic treatment because the position of the frenum can interfere with the closing of space and its removal can increase stability and improve success of the final orthodontic result.
Some adults require frenectomies when they are receiving dentures. The position of a frenum can interfere with the proper fit of the denture.