GNSS spoofing is an attack that feeds the drone FAKE satellite signals, so it believes it is somewhere it is not. The position on the map suddenly jumps kilometres away, the altitude reads wrong, and RTH flies in the wrong direction.
Spoofing is more dangerous than jamming because everything LOOKS normal — satellites are there, the map works, only the data is a lie. If the position or altitude behaves impossibly, trust your eyes, not the map in the app.