This is when your bring the front teeth on top edge to edge with the front teeth on the bottom. Is there a place where they form a tight fitting match. This is because you have worn the front teeth together by grinding over and over and have made a match between the upper and lower front teeth. The importance of this is it only happens when the bite on the b,ack teeth are strained and the jaw muscles and joints want you to move forward from that strained position.
You slide your lower jaw forward on the backs of your upper teeth till they touch tip to tip. Unfortunately this is not a very stable position, even if it is more relaxed so you go back to making the back teeth touch (the overbite position) but this is strained so you slide forward and get relief but there is no stability there so you go back. Same as the wearing of front teeth due to a bad uncomfortable bite.