Simply put, a masternode is a server on a decentralized network. It is utilized to complete unique functions in ways ordinary nodes can’t. It can be used for features like direct send / instant transactions or private transactions.
Because of their increased capabilities, masternodes typically require a sizable investment in order to run. But this is where incentivization comes into play, as masternode operators are rewarded by earning portions of block rewards in whatever given cryptocurrency they’re facilitating.
No masternode is quite alike as each network has its own pros and cons, but with that said, every system approaches payouts in a different way. Some cryptocurrencies pay out rewards to masternode operators multiple times in a day, whereas other projects payout operators once daily. The benefit of this dynamic is that operators can still earn money and provide a service to the network without having to invest in expensive cryptocurrency mining gear.
Many cryptocurrency users who don’t know better assume that masternodes are an extension of Proof-of-Stake (PoS) coins (i.e. cryptos that aren’t mined, but are staked), but that’s not true. There are Proof-of-Work (PoW) projects that make use of masternodes, so these kinds of nodes aren’t exclusive to PoS or PoW.
With that being said, running a masternode is PoS-like, in the sense that you generate passive income through a masternode just by holding your coins, similar to how stakers earn in PoS systems.