Most casinos allow an ace to be played as either a 1 or 11, whichever value is most beneficial to the holder. When a player has an ace that is being used as an 11, it's referred to as a "soft hand". These hands are played differently as the player cannot draw to a soft hand and bust (the player just revalues the Ace to count as 1 instead of 11).

The basic strategy for playing soft hands is to double down when the dealer has a high probability of busting (low value face-up cards).

In the event that you are dealt a card that changes your hand from a soft hand (Ace = 11) to a hard hand (Ace = 1) then change strategies to the hard hand rules discussed previously.

More

Blackjack game