From OpenCog

A type of Link which defines an inverse Hebbian connection that works in both directions.

Inverse Hebbian links connote negative associations between their arguments, i.e. they connote that the entities represented by the two arguments do generally NOT occur in the same situation or context, for instance:

HebbianLink dead breathing

The symmetric aspect defines that the association goes in both directions. When observing 'dead', 'breathing' is often not observed and vice versa.