A full melt pool means the wax has been heated long enough to reach the edge of the container. In a properly wicked candle, you should see the melt pool grow about one inch per hour. A full melt pool should be 1/4" to 1/2" deep which will allow the fragrance to disperse more completely into the room. Check out this article for help conducting a burn test to ensure that your candles are burning properly.

Candle Wax Melt Pool