The way I've read it (and how it was applied to me at the time) was a promise to ask you to marry in the future. I got one midway through college. Of course, said BF was emotionally abusive and we had a terrible co-dependent relationship, and I broke it off half-way through senior year and he said I could keep the ring.
Less than a month later, during a drinking session with friends (or so I'm told, I had friends who were dating these friends) he had this "epiphany" that I had been cheating on him for YEARS, so he snapped, turned into a complete stalker, destroyed a window at a friend's house while ranting about me, would call for hours on end, leaving terrifying messages on my machine, and demanding the ring back. More than once he stood bellow the balcony of my apt while I had all the lights off and yelled for hours demanding the ring. I finally sent it back to him through my then boss.
So yeah. Promise rings. Of course, I've not gotten any promise or engagement rings again. *sigh*