Doubling your bet and taking exactly one more card.
Ds means stand if you can't double. The basics: double hard 11 against 2–10, hard 10 against 2–9, and hard 9 against 3–6.
At the table
A player whose first two cards are 6 and 5 (11) adds a bet equal to the first one and says "double." The dealer deals exactly one card, often sideways, and the hand is over.