Double chin is the condition where deposition of fat combined with sagging of the skin under the jaw makes a swelling with the appearance of a second chin.
The appearance of a double chin is a common indication of weight gain, obesity, etc. These factors include weight gain and obesity which causes deposition of fat under the jaws, loosening and sagging of the skin due to age, poor walking and sitting portion which causes the skin to lose elasticity and even genetics.
Double chin by itself does not pose significant health issues. It may be an indicator of weight gain and obesity.