"Selling Stockholders" offerings

I don't recall having a position in a stock that had an offering where they "sold stockholders" ... I understand what it is, but don't feel like I fully understand what it could mean or WHY ... Hoping to have a discussion.. Here is the PR:

Paycom Software, Inc. ( PAYC ) (“Paycom”), a leading provider of comprehensive, cloud-based human capital management software delivered as Software-as-a-Service, announced the pricing of the underwritten secondary offering (the “Offering”) of 4,500,000 shares of its common stock by Welsh, Carson, Anderson & Stowe X, L.P. and WCAS Capital Partners IV, L.P. (collectively, “WCAS”), certain of Paycom’s executive officers and certain other selling stockholders (collectively, the “Selling Stockholders”). The price to the public in the Offering is $37.95 per share. Certain of the Selling Stockholders have also granted the underwriter a 30-day option to purchase up to an additional 675,000 shares. Barclays is acting as the underwriter for the Offering. The Offering is expected to close on or about September 21, 2015, subject to customary closing conditions. Paycom will not receive any proceeds from the Offering.