Bitcoin was the first blockchain, but it serves a single purpose: digital currency. Ethereum is the second most popular blockchain (by market cap), and it bills itself as "general purpose". Koinos is even more general purpose, but what does that mean?
Blockchains are really just an application that is designed to run on multiple computers (nodes). The nodes talk to each other and do *something* on a distributed ledger (read: database that every computer keeps a copy of). There are systems built into the blockchain application to ensure any given node can trust the overall network even if there are some nodes trying to mess everything up.
Bitcoin only does one thing on the distributed ledger: keep track of who owns how many bitcoins.
Ethereum is designed to run just about any code (smart contracts) on the blockchain. Developers create smart contracts (or dApps) and deploy them onto the network in the same way you would make and release an iPhone app or a website. Once it's deployed, people need to choose to run your app.
Koinos has the same goal but without the gas fees that everyone on Ethereum loves to complain about. Additionally, you can take the Koinos blockchain code, modify it, and create a new blockchain network. The mainnet is going to be one Koinos blockchain, but there will likely be multiple Koinos-based blockchains over time (maybe some general purpose and some single purpose). This is the piece that makes Koinos more than just a general purpose blockchain... it's a general purpose blockchain framework.
P.S. Up next: you might be wondering how that translates to the value of BTC, ETH, or KOIN. Why are these currencies worth billions?