Solitaire is easy to play and there are many variation of solitaire. This one is the Klondike version.
In the top-left, there's the deck. When you click the top card, three top cards are taken off and placed face-up next to the deck. You can drag these onto another stack. By clicking the deck again, new cards are revealed and this continues until the deck runs out. Another click will turn over the pile of open cards to form a new deck.
The four empty slots on the top-right are the stacks where you want all the cards to be in the end. These stacks need to be sorted from low to high (starting with the Ace), with one stack for every suit. You win the game when all four stacks are completed with their respective King.
The seven stacks at the bottom are temporary card holder. You can form stacks of cards here, as long as long as they follow these rules: Stacks need to be from high to low. Cards in the stack need to have alternating colors. Only Kings can be placed on empty slots.