Here's a proper def. of the situation... Not Random's guess Why Does a Shelf Offering Matter?
Shelf offerings give the company the flexibility to get the paperwork out of the way now and then offer the shares only when it needs the cash or only when the market conditions are good. They also signal to investors that the company intends to raise capital for new projects, possible acquisitions, or maybe to refinance something. However, companies aren't the only entities that can do shelf offerings; they can also involve founders or other managers (such as venture capitalists) selling all or a portion of their stakes in a company. Shelf offerings thus give these shareholders a way to monetize their positions.
Shelf offerings can dilute existing shares considerably if the offering comes from the company because new shares are being created. Selling a large volume of shares all at once can exert downward pressure on the stock's price -- a situation that is exacerbated when the stock is already thinly traded.