What important feature on the same map does Elrond point out to Gandalf and Thorin?

Elrond reveals that Thorin's map has a secret message,
written in moon letters, a type of rune-letters that only can be seen with "a moon of
the same shape and season as the day when they were written" (50).  Moon letters were
invented by the dwarves who wrote them with special silver pens. Elrond interprets the
message for Gandalf and Thorin, who are completely surprised, and a little "vexed," that
Elrond discovered this secret to the map.  The mysterious moon letters tell of how to
access a secret door to the Lonely Mountain; the keyhole will only reveal itself on
Durin's Day.  Elrond's important discovery will play a major role in aiding the dwarves
and Bilbo's entry into the Lonely Mountain.