The W, A, S, and D keys or WASD keys on the computer keyboard are used in place of the arrow keys. They are used because they allow the gamer to access more of the keys around them, which means more keys may be assigned to other tasks in the game. For example, you could use Ctrl to crouch.
How to use the WASD keys
With the WASD keys, W is up, A is left, S is down, and D is right. The pinky finger is used for keys to the left (e.g., Tab and Ctrl), ring finger on A, middle finger on either W or S, and index finger on D.