The Supplemental Register is an option for marks which are "merely descriptive" - that is, marks which describe the goods or services which they represent. For example, if an attorney were to trademark the term, "Patent Draft" and the attorney is doing just that, this would be describing the services represented. On the other hand, if a beer company were to register "Patent Draft" for a specific type of beer, it is not immediately apparent that this name refers to a product line of draft beers, and the name "patent" with regard to a type of beer if more than just merely descriptive. (This is by way of example, of course. A an examiner at the U.S. Patent and Trademark Office might disagree.)
The only requirement for registration on the Supplemental Register is that a mark must be capable of distinguishing goods or services (not that it actually serve such a function). Registration on the Supplemental Register does not confer any additional rights on the holder of a mark beyond those provided by common law. Marks registered on the Supplemental Register are not subject to opposition proceedings, but they may be canceled anytime by a court. Holders of such marks are still permitted to sue for trademark infringement based on common law rights (e.g. similar to if there were no registration). However, by placing a mark on the Supplemental Register, it puts adversaries on notice and after five years of use, the mark is deemed to have "secondary meaning" and be registered on the Primary Register like any other trademark.
(With thanks to Wikipedia.)
In United States trademark law, the Supplemental Register is the secondary register of trademarks maintained by the United States Patent and Trademark Office. It was established in 1946 by Sub-chapter II of the Lanham Act, for the purposes of allowing domestic registration of trademarks which do not meet all of the requirements for registration on the Principal Register, so that the holders of such a mark could register it in another country. This was necessary because under the Paris Convention for the Protection of Industrial Property, foreign registration was not permitted in the absence of domestic registration, and the trademark laws of countries outside the U.S. often have less stringent registration requirements for marks.