You can keep fighting even after you should be dead.
Benefit: Once per day, you can gain a number of temporary hit points equal to your Constitution modifier. You can activate this feat as an immediate action when reduced to 0 or fewer hit points. You can use this feat to prevent yourself from dying. These temporary hit points last for 1 minute. If your hit points drop below 0 due to the loss of these temporary hit points, you fall unconscious and are dying as normal. If you also have the ferocity racial trait, you can use that once you have lost the temporary hit points from this feat.
Source: Advanced Player's Guide