A shelf registration is used by companies that want to sell shares to the public, but not immediately. With a filing "on the shelf" with the SEC, the company has up to two years to sell the shares.
So-called shelf registrations are allowed under SEC Rule 415, which lets companies get prepared so they are ready to offer shares (or other securities, such as bonds) when market conditions are favorable or when money is needed urgently.
The unissued shares are held by the company and are known as treasury shares. As they are not outstanding, these shares are not used in the calculation of numbers like earnings per share or "fully diluted" earnings. However, the shelf registration alerts a savvy investor that the company in question may issue all or part of these shares in the near future.