What's that device on a pole the man is holding in the field along with a phone?
Surveying marker. There's a laser device somewhere else measuring precise distance to that marker
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.