Twice as Much Fun
Doubling down means doubling your wager and taking one and exactly one more card. This is an advantageous option for the player, because it allows you to change (increase) your bet after seeing your hand and the dealer's upcard.
Some casinos only allow you to double down on 10 or 11, but other casinos allow you to double down on any hand.
Some casinos allow you to double down after splitting a pair; other casinos do not.
To signal to the dealer that you want to double down, put your cards down in front of your original bet and place an additional wager BESIDE your original bet. Do NOT place your bet on top of your original wager.
Blackjack basic strategy will give you guidelines for when to double down and when not to.