It basically lets them have direct mind to mind communication, which also allows people (like Sauron) to attempt to dominate the other directly. Even somebody like Denethor who wasn't corrupted was swayed by being showed Sauron's plans and made him deathly paranoid.

This channel makes great videos on LotR and I highly suggest watching it.

The Palantir were created by the men of Numenor (basically Atlantis) which fell due to corruption. They then took those to the mainland and installed them at key locations.

It basically lets them have direct mind to mind communication, which also allows people (like Sauron) to attempt to dominate the other directly. Even somebody like Denethor who wasn't corrupted was swayed by being showed Sauron's plans and made him deathly paranoid.

This channel makes great videos on LotR and I highly suggest watching it.