The phrase ‘stopping down’ describes the act of adjusting the aperture to a smaller opening. In practice this means increasing the f-number on your camera.
Stopping down from f/4 down to f/11 would be changing the aperture from a wide aperture to a fine aperture. This big change would involve stopping down through a number of stops. It would have the effect of a considerable increase in the depth of field.
As shown by the simple example above the change in aperture has an impact on the depth of field as well as decreasing the amount of light allowed to reach the digital image sensor. Stopping down, and its opposite, increasing the size of the aperture, should therefore be considered one of the main ways that exposure can be controlled.