Trademarks can be registered on the Principal or Supplement Registers. Strong, distinctive marks are registered on the Principal Register while weaker trademarks, such as merely descriptive marks, or marks that are no inherently distinctive, can be registered on the Supplemental Register.
The Supplemental Register does not offer the same level of trademark protection as the Principal Register, but it does have some advantages for marks that would not otherwise qualify for registration on the Principal Register. Often the USPTO will refuse a mark on the Principal Register for being descriptive, but allow the owner to move the mark to the Supplemental Register.
In short, the Supplemental Register is a weaker form of trademark protection, but may be the best alternative if registration on the Principal Register is not possible.