A Pool of Imaging is a spell from the White Winds School that allows people to converse at a distance, a bit like a magical videophone. The spell uses scrying bowls as the conduit with each mage's image appearing in the other's bowl as if a tiny person was standing within. The image also throws a flickering illumination like a television screen.
Only Master- and Adept-class mages can use the spell. It's mostly only used in peacetime as it drains the mages using it, is incredibly obvious to any other mage in the vicinity lighting up like a magical beacon, and leaves both mages open to attack from outside forces. Because of the dangers inherent in using the spell, mages try to keep their non-magical employers from finding out about it.
Kethry uses this spell in By the Sword to talk to her son Jendar about the situation in the Rethwellan royal court and council following the death of King Megrarthon.