A Shareholder Secondary is a type of Secondary Share Sale where the Company whose shares are being sold is not directly managing the transaction. Sellers in these transactions are typically early investors or (ex-)employees with vested shares who want to exchange shares for cash before a full exit event.
In a Shareholder Secondary the issuing Company is likely to only be involved at the end of the process, as depending on transfer restrictions the Company may have to approve the transaction, and the transaction will have to be reflected in its member register.