According to its light paper, Polkadot aims to be a "next-generation blockchain protocol that unites an entire network of purpose-built blockchains, allowing them to operate seamlessly together at scale." I know that sounds like a mouthful, and since this is an ELI5 article, we can simplify that definition just a bit. If you only take one thing from this article, remember that Polkadot is a way of allowing different blockchains to work together. In reality, Polkadot has a very ambitious mission and therefore a lot of moving parts. Since this is an ELI5 post, I'll do my best to give you a basic understanding of what Polkadot does and high-level overview of how it works without getting too detailed.