Thin Blue Smoke (TBS) is a light, almost invisible blueish smoke. It is the goal for making the best tasting BBQ on a smoker burning wood and/or charcoal.
Too much white smoke means creosote will be coating your BBQ meat and imparting a bitter flavor.
Achieving thin blue smoke is not difficult but does take some practice. As you start the smoker with wood or charcoal the white smoke will be billowing out. As the fire reaches the desired temperature you can control the burn by adjusting the air flow. Not until the target temperature is reached, the white billowing smoke has disapated, and the thin blue smoke has appeared should the meat go on the smoker.