This phrase presumably originated as a serious warning, and can still be used seriously, to mean exactly what it says. However, it is now often used jokingly or semi-jokingly -- still usually when something dangerous or difficult is being attempted, but in the knowledge that the expression has become a cliché, and with no real concern that anyone really would try it either at home or anywhere else. The serious meaning could be phrased in a number of ways, I guess, but there's no other expression that fits the joky meaning (not that I'm aware of, anyway).