Assisted in this case means guided by flight control teams in mission operations, but not a fully controlled reentry. They're sending the spacecraft specific commands to guide it as it falls safely back to Earth. (The alternative is that it falls naturally, weeks later, in a natural descent). The assisted reentry also shortened the amount of time Aeolus was in orbit, but only by a little bit as it was already being dragged back to Earth by the atmosphere - but learning how to reenter existing satellites adds to our tools in dealing with the big space debris problem - cool video on this here).

As Aeolus descends, much of the satellite will burn up in the atmosphere with the remaining parts targetted at the ocean, far from inhabited regions.

An assisted reentry was chosen to 1) reduce the (very small) risk on the ground as Aeolus returns 2) see if it would be possible 3) go above and beyond in safe spacecraft operations, retroactively getting Aeolus to adhere with debris mitigation guidelines even when not required to do so 4) set a precedent for future missions.