Uses four values.
One for horizontal offset, one for vertical offset, another for blur, and the last one for the color of the shadow.
Uses five values.
One for horizontal offset, one for vertical offset, one for blur, another for blur distance, and the last one for the color of the shadow.
Shadow inside the box
Do one of the two options, but put inset before the values to put the shadow inside the box.
One Example
Using 40px and both vertical and horizontal offsets, 20px blur, with a red shadow.
Border-radius at 30px.