DNS (the Domain Name System) is the internet's address book. Every device on the internet is ultimately reached by a numeric IP address, but people don't want to memorize strings of numbers, so DNS exists to translate a human-readable domain name into the correct IP address behind the scenes, every time someone visits a site or sends an email.

When you type a domain into a browser, your device asks a series of DNS servers "where does this domain point?" until it gets a definitive answer, then connects to that address directly. This all happens in a fraction of a second, invisibly, before a page starts loading.

DNS isn't one server or one company; it's a distributed system, which is part of why it's resilient but also why changes can take a little time to be visible everywhere at once. See Why DNS Changes Take Time to Go Live.