A ‘sop’ was a kind of milk soaked bread. It signifies giving something of little value to someone by way of granting concession (yielding to a part of something) or appeasement (pacification)… to give them a bribe of not much monetary value to quieten them down or distract them.
You have obviously looked it up yourself, as Cerberus was not mentioned in your original quote.
Cerberus (sometimes seen as Kerberos) is a mythological monster from ancient Greek story. He was a three-headed dog. He guarded the entrance to Hades (the infernal region of the dead) and Greeks would place a ‘sop’ in the hands of their dead, so that they may give it to Cerberus and ease their passing into the underworld.