Is it referring to the ethereum blockchain?
Sorry if this seems too basic of a question. I'm new and I can't seem to find an answer anywhere.
You can think of
window.ethereum as an object that let you communicate with a browser extension(in most cases Metamask). Actually, it literally is an object.
The browser extension(let's say Metamask) does all the hard work of managing user's addresses, and interacting with a blockchain. But there is no way for your web3 application to directly talk to Metamask.
window.ethereum sits in between a web3 application and Metamask. It lets your web3 app to interact with blockchain through Metamask.