A polar relay is one in which a permanent magnet is used to hold the contact arm in one position or the other, while current flow in the coils is used to switch the contact between the two positions.  Current flow in one direction will close the contacts and current flow in the other direction will open the contacts.

Basically, telegraph systems fell into two classes.  The single-current system relied on current flow and lack of current flow to identify the two states.  The double-current (or polar) system relied on current flow in both directions to identify the two states.  The polar relay was a necessary component of the double-current system, since it could respond to the current reversals.  Typically, a polar relay was used to operate a standard sounder at a receiving station.

The polar relay had a significant speed advantage over a standard relay which relied entirely on a spring to return the contact to the de-energized state.