Burning Pitch is a Technology researched at the Fortress which does damage over time to any unit it hits. Burning pitch damage goes through all armor, making it particularly effective against units which have been upgraded in the Armory.
In the metagame, Burning Pitch is one of the strongest features of the Norse civilization. In fact, many in the community believe that it is over-powered and in need of a patch. Because Age of Empires Online is no longer in active development however, no balance patches addressing Burning Pitch are likely in the foreseeable future. To compensate for this, high-level PvP tournaments and events sometimes ban the technology.