Every time I close my mouth this simple thing forms in my chin. What is it and how can I fix it? (Photo)

This is like solving a puzzle? A crease may be related to dynamics of your lower lip(?) muscles. Try to avoid lip/tonque motion that creates it. Or, if deformity is there all time one can consider to fill it with fat graft or soft tissue filler.It would be in your best interest to seek a consult of a board certified plastic surgeon for detailed evaluation. Good luck. 

This is due to mentalis muscle contraction (forms a ball) as you close your mouth or bite down. This can likely be improved by an intraoral mentalis muscle release. You can also have Botox injected which should create a mentalis muscle relaxation of this movement.

It looks like a bunching up of he mentalis muscle.If your chin is receding, an implant may correct this.Otherwise Botox injections given several times a year may stop the muscle activity