Someone that appears attractive from a distance whom upon further inspection turns out to be totally hot often causing others to stop and marvel at their beauty and/or hotness.
The term is derived from the motions of which you would take when approaching a stop sign in a car. You slow down to check said person out and when you come to the conclusion that they are indeed worth the effort you come to a complete stop and approach.
The opposite of a speedbump.
Trey: That sucks for you cause it looks like I'm pulling up to a Stopsign right now.