You are more resistant to massive damage.
Prerequisites
Constitution 13.
Benefit
Your massive damage threshold increases by 6, you gain a +1 bonus on the Fortitude save to resist death by massive damage, and you add 3 to the number of negative hit points you can accrue before dying.
Special
Without this feat, you have a massive damage threshold equal to twice your current Constitution score, and you die if you have negative hit points equal to your Constitution score. With this feat, your massive damage threshold is twice your current Con + 6, and you do not die unless you have negative hit points equal to your Con +3.
You can gain this feat multiple times. Its effects stack.