Your two card total is in the far left-hand column. The top row is the dealers up card. All your decisions are based on this information, the chart will tell you whether you should Hit (H), Stand (S), Double (D), or Split (P).
To double down simply means you can double your original bet after you receive your first two cards. If you decide to double down you will then receive only one more card. To split a pair simply means that when you receive any pair on your first two cards, you can split them into two separate hands. You can then play each hand as individual hands. After splitting, follow the chart for the hand you are playing.
The following chart is the Single Deck Master Blackjack Strategy Chart. When you can find a single deck blackjack game available you should always choose it over a multiple deck game.
The following basic blackjack strategy chart is the Multiple Deck Master Blackjack Strategy Chart. Multiple deck blackjack is the game that you will find most often. The strategy on multiple deck vs. singled deck is almost identical. Their are only 9 differences between the two strategy charts. Las Vegas rules allow you to double on any two card total. When you find a blackjack game that lets you double on any two card total then you are playing Vegas rules - you don't necessarily have to be in Vegas to find these rules in effect.
The following Multiple Deck Master Blackjack Strategy Chart is based on Atlantic City rules. Atlantic City rules allow you to double after splitting. Because you are allowed to double after splitting the following chart contains a more aggressive splitting strategy.