The pass through the mountains of the Pelóri that led from the shores of the Great Sea westward into Valinor. Its name translates as 'Pass of Light', a reference to the light of the Two Trees that shone through the pass in the days before the Sun and Moon. The Elven city of Tirion stood between the walls of the pass, and Eärendil came there on his famous journey into the West, as told in Bilbo's song quoted above.
The much more common form of this name is 'Calacirya', and in fact the particular form Calacirian only ever occurs in Bilbo's Song of Eärendil. Throughout The Silmarillion, and even elsewhere in The Lord of the Rings, 'Calacirya' is the preferred form, suggesting that the modification to 'Calacirian' is purely for Bilbo's poetic purposes.