A telescope is a light trap, which bundles all the collected light together and produces an image. With Newtonian telescopes, however, a secondary mirror deflects the light to the side into the focuser, because otherwise you would not be able to observe with it. Other designs are also obstructed by a secondary mirror.

The secondary mirror in optical systems creates a shadow which reduces the contast and effective aperture performance. Every component in the optical path of a telescope creates this shadow, known as obstruction. This is basically the case for all reflector telescopes (the exception is the 'Schiefspiegler'). Only a refractor has 0% obstruction, since no components disturbs the optical path.    

In the illustrations you can see that the image is diffracted by an obstruction, resulting in a loss of contrast. The larger the secondary mirror, the greater the obstruction. 

To find the contrast performance, or the contrast loss, of a telescope you simply deduct the diameter of the secondary mirror from the diameter of the primary mirror. This gives you the effective contrast aperture that a telescope without obstruction would have.