It was keyworded in the 2012 Core Set. Hexproof literally means that the creature can't be the target of spells or abilities your opponents control. You can still target your own creature with hexproof ( for example with cards like Giant Growth and Holy Strength), but youropponent can't target them with spells such as Doom Blade.
An ability similar to hexproof is shroud, except that shroud prevents you from targeting the creature as well.
Creatures with hexproof (and shroud) are still susceptible to cards which effects that target "all creatures", such as Supreme Verdict.
