A wet spot refers to the patch on your glass that almost looks like an air bubble between the wax and glass. What in fact has happened is the wax has pulled away from the edge of the glass. Once this happens it will not re-adhere. It does not affect the performance of the candle in any way but is an aesthetic thing.
It generally happens when the candle temperature fluctuates and the wax expands and contracts. You will find it very nearly happens always overnight if you leave your candles out. It can be controlled by using a good quality wax that has great glass adhesion, but also with your pour temperature.
If you find your candle wetspots as it is drying, try increasing the pouring temperature. If your candle wetspots overnight, make sure you put the candles in a warm place & don’t leave them exposed to the cold.
Sometimes however no matter what you do you will not prevent wet spots. The climate changes both outside and inside, even during transportation or the air-conditioning in a shop can all cause wet spots.