A secondary market offering, according to the U.S. Financial Industry Regulatory Authority (FINRA), is a registered offering of a large block of a security that has been previously issued to the public.
The blocks being offered may have been held by large investors or institutions, and proceeds of the sale go to those holders, not the issuing company.
[1] A secondary offering is not dilutive to existing shareholders since no new shares are created.
The proceeds from the sale of the securities do not benefit the issuing company in any way.
The offered shares are privately held by shareholders of the issuing company, who may be directors or other insiders (such as venture capitalists) who may be looking to diversify their holdings.