Your Titan will lose health at a slower rate when doomed, allowing you to fight longer before having to abandon the Titan.
— In-game description.
Survivor is a Tier 2 Titan Kit that appears in Titanfall. This kit makes titans lose health three times slower when doomed, both from time and damage. This can be interpreted as increasing the maximum time in a doomed state from 4.5 seconds to 13.5 seconds, and increasing doomed state health from 3750 to 11250. It allows to stay in titan for longer periods of time to deal more damage and, possibly, avenge its state. Note that doomed titans are still vulnerable to termination.