Foul Hand
A foul hand is a hand that contains the wrong number of cards, and so cannot be played.
The simple explanation
If you should have two cards in Hold'em but you hold three or one, your hand is foul.
A closer look
Players should count their cards and keep them in view. If you notice an extra card, call the dealer at once. A foul hand is dead, and the player has no claim on the pot. The rule protects the game from errors and from cheating, because a player who is allowed to keep an extra card would have an advantage.
A practical example
A player finds that he holds three hole cards in Hold'em. The dealer declares his hand foul.
Full explanation
A foul hand is a hand that contains the wrong number of cards, and so cannot be played.
A foul hand has the wrong number of cards.
It is dead and cannot win.
Count your cards and call the dealer.
The rule protects the game.