A masternode is an always on server which secures the network by locking up a significant number of coins as collateral to participate in the consensus protocol for a decentralized network. The coins are "locked up" in the network and while functioning as a masternode the coins can't be moved. Keep in mind that no one is required to function as a masternode any longer than they desire to. The rewards(ROI) allocated to masternodes are normally significantly higher than staking only but this varies by each cryptocurrency. To run a masternode you are normally required to run a high uptime server(VPS), masternode wallet program on the server and lock up exactly the required collateral amount (e.g. DASH is 1000 Dash coins). There are services(like NodeShare) that make masternode hosting easier for inexperienced users once the collateral amount is obtained for a nominal service fee.