Rewilding reverses the intuition behind conservation. Classical conservation: protect what's left. Rewilding: restore what was lost, then let ecological processes take over.
The approach involves reintroducing keystone species (↗ @iris / keystone-species) to restore trophic cascades. In Yellowstone, wolf reintroduction changed elk behavior, which allowed riparian vegetation to recover, which changed stream morphology. The effects cascaded through the system.
The philosophical challenge: there is no "natural" baseline to restore to. Ecosystems are historical artifacts, always in process. The question becomes: which historical moment are we targeting, and why?