Camlann is where Arthur Pendragon made his stand, and where the great battle between the Saxons and the Knights of Camelot fought. Mordred met his end upon Camlann, along with a lot of Saxons and Camelot knights. Arthur was mortally wounded on Camlann, but two days later Arthur died near the shores of Avalon.
The plain of Camlann is seen in Morgana's dream, but only in a vision. Morgana cried out to Merlin, in his Dragoon disguise saying, "Help me Emrys, please," (The Darkest Hour: Part 1).
Camlann was also seen by Merlin, when he saw a vision of Mordred killing Arthur (Arthur's Bane: Part 1).
Camlann is only mentioned in the prophecy that Finna gives Merlin, stating that the great battle will take place on Camlann, also where Arthur will meet his end. This is when Merlin finds out where the war will begin (The Kindness of Strangers).
The Great Battle
Camlann makes its final appearance in the finale, where Arthur decides to make his stand for the war. Merlin tells Gaius about Arthur's decision, stating that this battle is the only way he knows how to save his people. Gaius says to Merlin that he shouldn't let Arthur go to Camlann, since Arthur would die there, but Merlin says that Arthur won't listen. While Merlin is trying to get his magic back, he is trapped in the Crystal Cave from Morgana's doing, Arthur and the Knights of Camelot arrive at Camlann and set up their camps. The Saxons shortly arrive, and the war begins. Later, Merlin meets a spirit of his father, telling him to believe in himself. The next morning, Merlin gets his magic back, and comes out from the cave as Dragoon the Great. Merlin gets to Camlann, and helps the Knights of Camelot defeat the Saxons. While Merlin is away coming down to the ground, Mordred manages to wound Arthur, but dies later at Arthur's hand. Merlin comes down to see the unconsious Arthur, and carries him away. This is Camlann's final appearance.
In the legend
Camlann was where King Arthur and Mordred waited to fight, since a deal between them was made. If one person from either side should draw their sword, the war would begin instantly. An adder came over to one of Mordred's knights, and stung him on the heel. The knight drew his sword and slew the adder, and the war begun. Every single person on the battlefield died, except for Sir Bedivere who took Arthur to Avalon. Camlann is mostly known in the volumes Le Morte d'Arthur, by Thomas Malory.
Camlann has a number of possible locations. The plain is said to have been near a crooked river, and the prefix "Cam" means crooked. Camlann could be near the Slaughter Bridge in Cornwall. Wace, like Geoffrey, speaks of this site in Cornwall as the final battle. Fort Cambloganna (Castle Steads) on Hadrians's Wall. This was one of several Hadrian's Wall forts that saw heavy fighting during the Caledonian invasion. It does not fit with the earliest historic references which refer to Cornwall. Camlann could possibly be near to Glastonbury Tors, which where Arthur was believed to be buried.