Rewilding means to take an area that was previously built on and controlled by humans and get it back to a wild natural place. For example, let’s say that you have an area that used to be an industrial area, with a bunch of warehouses that are old and unusable. Instead of tearing those buildings down and building new ones, you decide to remove the buildings and restore the nature by planting native species. You might need to do a bit of terraforming to get that land back to what it was. The goal is to let nature take over the land and manage itself. Unlike a park that is planted and organized and controlled by trimming plants, watering them, and creating paths, to rewild an area is to not manage it and just let it be wild.
Humans have taken over a large amount of land for our own use and have left only a bit of it wild with some national parks. The goal is to build our communities denser and leave more surrounding areas wild so other species can also exist. Certain cities, like Portland, are modifying their regional plans to rewild areas of the county and remove human control and buildings instead of building out and over more land.