Bitcoin was the first cryptocurrency and is currently the largest one by total market capitalization, collectively worth over $1 trillion as of 2021.
Bitcoin is the world’s first completely open payment network which anyone with an internet connection can participate in. Bitcoin was designed to be used on the internet, and doesn’t depend on banks or private companies to process transactions.
Similar to how a bank tracks assets, Bitcoin tracks who owns what using a blockchain. What sets the Bitcoin blockchain apart from a bank's ledger is that it is decentralized, meaning anyone can view it and no single entity controls it.
Understanding Bitcoin is key to understanding the rest of crypto, since all other cryptocurrencies share many of the foundational aspects of Bitcoin. Here are a few articles that best motivate Bitcoin, explain how it works and why it is important.
I highly recommend the Bitcoin Whitepaper, published by Satoshi Nakamoto in 2008 introducing Bitcoin to the world. It is fairly short (9 pages) and contains all the technical details of Bitcoin's implementation. Bitcoin: A Peer-to-Peer Electronic Cash System