All 6 cards of one of these piles are to be placed face up on the table. The remaining cards are in a pile which is available and visible to both players. If the sum of the cards face up on the table is less than or equal to 21 then no legal move is possible so you must shuffle the cards and deal a new game.
Players alternate moves.
A move consists of exchanging a card on the table with a lower card from the other pile. The player whose move makes the sum of the cards on the table under 21 loses.