What is this thing; what's it for, what does it do, how's it used exactly, etc.?
It's a radar intended to observe nearby boats.
tweet by NASA Spaceflight Reporter Michael Baylor:
SpaceX has asked the FCC for permission to operate a Garmin radar on top of a water tower in Boca Chica.
The "water tower" looks familiar.
FCC filing: Space Exploration Technologies Corp. 0459-EX-CN-2020:
...d) List any natural formations of existing man-made structures (hills, trees, water tanks, towers, etc.) which, in the opinion of the applicant, would tend to shield the antenna from aircraft:
This radar will be attached to the top of a water tower and is masked between 164 to 290 degrees to avoid illuminating launch or test vehicles. Please note that a TFR will be in place when the radar is operating.