Answer: You'll be sorry you asked (full details here). It's an attempt to apply Sabermetrics to Survivor. (Sorry yet? If not, read on.) Essentially, an attempt to answer: "Who's the best Survivor player?"
Survival Average is a simple sum of fractional Challenge Wins (ChW), weighted TC Ratio (wTCR), and (eventually) a weighted Jury vote% (JV%).
ChW: Challenge wins. This includes tribal challenges (worth 1/# of participants, zero points for the win if you sit out), individual challenges (worth 1 point), and Redemption Island duels (worth 0.5 points). So your score can still go up on Redemption Island.
wTCR: weighted Tribal Council Ratio. This measures how in-the-loop you are on the boot, and punishes you if people vote against you. As it should. It uses the # of times you voted for the person who was booted (Votes for Boot, VFB), # of times you were voted against (Votes Against Player, VAP), and total # of times at tribal council (Tribal Council Appearances, TCA). Tough luck if you end up in the non-eliminee-voting group of an intentional vote-splitting scheme. Valar morghulis. The formula is as follows: wTCR =2* [VFB / (4+VAP)] x (14/TCA).
But wait! It gets worse! Full scoring and more detailed explanation now available in the Survivor 23 Box scores section.