With each level of this Perk, you will get +20% bonus when checks are made against the Outdoorsman skill, for purposes of determining if the player receives dehydration damage (Desert random encounters) or damage from a rockfall (Mountain random encounters).
The Outdoorsman skill itself does not increase.
This perk doesn't exist in the G.E.C.K. but an image of a Vault Boy named "perk_survivalist" can be found in the game files and proves that the perk was planned. Otherwise, benefit, prerequisites and description are unknown.
The image for this perk in Fallout, Fallout 2 and Fallout Tactics is obviously inspired by Sylvester Stallone's character John Rambo, from the movies First Blood; Rambo: First Blood, Part Two; Rambo III and, more recently, Rambo.