A Shared Masternode is a Masternode managed by Goldnode where participants can get in on a share of the rewards. The masternode is divided into seats (most often 100 seats) and Goldnode have a Initial Masternode offer with a deadline to sell all these seats. By participating in a IMO (Initial Masternode Offering) and buy seats you get a share in the Shared Masternode.
So instead of having all the assets to start your own Masternode, Goldnode offers you the service to share a part of a masternode with others - and by this effectivly lower the entry point.
Goldnode also manages and monitors the operations of the masternode to make sure it runs smootly.