"Lord Voldemort placed the Dark Mark brand on his followers' inner left forearms both as a sign of their loyalty to him and as a method of summoning them to him when he desired. This was done by Voldemort pressing his wand to any Death Eater's Mark, which turns it jet black and delivers a burning sensation to any person with the Dark Mark. Feeling the burn, the Death Eaters then Apparate to Voldemort's side."
the death eater's mark is burned onto Voldemorts supporters, whom he trusts the most. When the dark mark is casted into the sky, the death eaters begin to feel the burn, which is a sign that their lord needs them, or has returned, or they are trying to scare the witches and wizards.
