SHL command

okay..
I have a little problem understanding the SHL command.
Let's take an example:

mov bx, 2
mov dx, 2
mov cl, 8
shl bx, cl
mov cl, 6
shl dx, cl
add dx, bx

now, the value of dx is 640. (if the example is correct)
But I don't clearly understand what does the SHL command actually
do. I know that it move bits but that is the difficult part.

So, could anyone give me explanation for the example above? Or if it does not make any sense then give me an example of your own (with explanations, please)...

Thanks.