"Darmok and Jalad At Tanagra" is a metaphor to fight a common enemy.
"Shaka, when the walls fell" refers to failure, to defeat.
They were used on Star Trek: The Next Generation episode "Darmok" when Picard was stranded on a planted with an alien who they couldn't communicate with. Just as "Juliet on the balcony" conveys romanticism, this race communicates in metaphors from their own stories and legends.
For more information, visit the Memory Alpha article on the Darmok episode