If you are drawing dead, it means you don’t have any chances of winning, even if your hand improves on the turn or the river.
For example, let’s say you have a JsJh, and your opponent has a 10c8c. You see 7c 9c Jc on the flop and are very happy to hit the top set, but you are drawing dead since your opponent already has nuts with a straight flush. You can’t draw any card to create a stronger hand, and even if you hit quads with the last jack in the deck, you would still lose the hand.