Masternode shares are for those that can’t afford to purchase the 1000 dash that are required to run a Masternode. Masternode shares allow users to accumalitively meet the 1000 dash requirement of a Masternode. This is achieved by utilising a 3rd party service that pools together the funds and sets up the Masternode for the shareholders. The service provider then divies up the Masternode rewards and pays it out to all shareholders of that masternode.
Remember that by purchasing shares in a Masternode, you are putting trust into the 3rd party service, so please do your own due dilagence before purchasing.