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 100 seats and you can buy 1 or more.
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.
Please note that you cannot dismantle the masternode if you do not hold more than 51 seats. If you want to dismantle / withdraw yoyr stake in a shared it is much more slow process. We need to initiate a voting with the other seat holders of your masternode and this can take many days to complete. So if you want to be sure to be able to dismantle/withdra you stake then aways buy minimum 51 seats in a shared. Or setup a full node.