What does a candle on top of a skull tattoo mean?

Well to me, the candle represents the life that is burning away and eventually it will reach the skull, which, as expected, represents the death. This is a symbolism for the fact that life will pass by quickly and there is are lots of things to do before it ends, wasting time will be regretted when there's no more time left... You Only Live Once, live it well ;)