It adds Hit point to your toons' stats. I am not sure, but I assume it works like Hit Point runes do. I have few defensive runes on my main and only champ gear so far. and have been told the reason my toon can take so many blows and get so many skill shots in is because I have outrageously high constitution on. But I am on my first server, so I can only assume that is why she kills much better equipped and runed players with better heroes. Anyone know for sure?