The phrase one eyed jacks refers to two of the jacks in a standard deck of playing cards: the Jack of hearts and the Jack of spades.
In a standard deck, the men drawn on these cards are drawn facing sideways, so that only one of their eyes is shown (hence, they appear to be "one-eyed"). The other two jacks are drawn facing the viewer so that both of their eyes are shown:
One eyed jacks is a phrase most often used to describe cards that are being declared wild. It is a convenient way to specify that half the jacks should be wild, and as an ancient part of poker slang, it's also just fun to say.
One eyed jacks are often paired with the "Suicide King" in declaring wild cards.