They tell you when to hit and when to stand based on the basic strategy rule. The green blocks mean you should hit, the red blocks mean you should stand. The blinking circle indicates what you should do. The colors on the strategy cards match the play buttons on the tool bar. There are also similar cards for Doubling Down and Splitting. as well as versions for soft hands.
Soft hands are hands that contain at least one Ace.
An Ace can have a value of 1 or 11, you decide based on the other cards in your hand. On a double down the dealer will give you just one more additional card. If a casino allows double down, you should do it if you have a card total between 9 and 11, with the following exceptions. The dealers card is an Ace and your total is 11. When your total is 10, double down unless the dealer has an Ace or Ten.