Arthur was hidden for his father's safety and his own safety from the Unseelie Court. His father, Uther was supposed to give him to the court as a part of the deal they made.
Merlin a wizard , took Arthur to a village to never be found by the Unseelie Court. He was taken as a child and never really knew his parents, Uther and Igraine. He was raised by this "father" Sir Ector.
Sir Ector knows about Arthur and when Ulric comes to their village Ector tells Arthur to make sure he does not tell anyone his true idenity. Ector tells him to be his brother Cie's squire if anyone were to ask who he was.