For each rank of this ability that you possess, you gain a +2 Bonus to Fortitude saving throws against any effect that would move you against your will or knock you Prone. This includes Shove and Trip attempts, as well as any spells or abilities that would deal damage in addition to moving you or knocking you prone.
This Bonus is also applied to Fortitude DCs and saving throws against effects such as Powerful Blow or Unbalancing Blow which use the same die roll to determine effects against the Defense and Fortitude DCs or saving throws. If you make a Defense saving throw against such an effect, you do not gain the Bonus from Immovable to your Defense saving throw, instead adding it to the die roll afterwards to see if the Shove or Trip attempt affects you.