A GNSS (Global Navigation Satellite System) receiver, it can produce your location down to a 10cm resolution (depending on which satellite system you use). The software on the phone presumably is the logger that lets you 'click' positions as you walk around the border of something.

It looks like this one (or a related product):

https://bench-mark.ca/product/juniper-geode-sub-meter-gns3-r...

Surveying marker. There's a laser device somewhere else measuring precise distance to that marker