Bitcoin is 2 things:

1. A digital currency
2. A decentralized platform

It was invented to be a peer-to-peer payment solution. It's main characteristic is that it allowed people to transfer funds without having to go through a centralized entity (like a bank). That was possible thanks to it's underlying technology, the blockchain.

I made this short video where I explain it in a bit more detail: https://www.youtube.com/watch?v=yaPeSJTUmQs&index=1&list=PLkosbfdomGrPVpQ0ynWnEnOItywY4wFMw&t=2s