Hello! That's a good question actually. Think of masternodes as an 'incentivized' full node. Now, see, we have Bitcoin right? And Bitcoin has a blockchain. What makes blockchain special is that it's immutable - it cannot be altered, changed, corrupted or hacked. That's because copies of this blockchain (which is basically a distributed database) are spread out or "propagated" among the nodes. Nodes are simply devices which have a complete copy of this blockchain. The problem with Bitcoin's nodes are that they're not incentivized - meaning, they're not paid or rewarded. Now if you have to download a lot of data - say 100 GB, that would cost you in electricity, time and bandwidth. So if you're not getting rewarded or paid to run a full node, why should you?

Then comes Masternodes. Basically full nodes that are rewarded. The nodes can be hosted on servers and you pay a little and get rewarded accordingly or how the coin distributes its rewards. There's more to it than that, but basically that's it.