Uther and Igraine were King Arthur’s birth parents.  When Uther first met Igraine (also spelled Igrayne), she was married.  She refused King Uther Pendragon.  He was devastated, and decided to go to war with her husband, the Duke.  He visited her disguised as her husband, and that was how Arthur was born.

When Igraine marries Uther, he asks her whose child she is carrying.  She admits that she is perplexed that the child seemed to have been conceived when her husband was dead.

I went unto bed with him as I ought to do with my lord, and the same night, as I shall answer unto God, this child was begotten upon me.  (ch 3)

Uther knows she is telling the truth, of course, because he was the one who pretended to be her husband and snuck in and bedded her. When he tells her this, she is thrilled because she is actually married to the father of her child.

When Arthur is born, Merlin takes him and gives him to Sir Ector.

So the child was delivered unto Merlin, and so he bare it forth unto Sir Ector, and made an holy man to christen him, and named him Arthur; and so Sir Ector’s wife nourished him with her own pap (ch 3).

In the end, Arthur is of course destined for greatness.  The strange circumstances of his birth are the stuff of legend for sure!