GNSS jamming is radio interference that drowns out the satellite band so the drone loses its position lock. The signs are characteristic: the satellite count collapses, the map greys out, and position hold and RTH stop working.
The correct response: switch to manual or ATTI mode, fly by visual references, land, and report the occurrence. In the Baltics jamming is a real, recurring phenomenon, so factor it in already when planning the flight.