what Gjallar means
Gjallar (or Gjallarhorn) is the horn which Mimer and Oden used to drink from Mimer's well of wisdom and intelligence. It is also the horn used by Heimdall, the guardian of Asgard, to warn that the giants are coming to fight the gods and the end of the world is near.
Think of it as a source of wisdown with the potential of warnings. :)