How long an object lives after it is destroyed. Normally an object's wreck exists for 10 seconds, then fades away in 2 more seconds. So, the default TimeToLiveAfterDeath is 12 seconds, and the default TimeToStartFadeAfterDeath is 10 seconds.
Setting this property to 0 makes the destroyed object immediately disappear. The Defgun uses it to stay around so its wreck stays visible.
Source and more details