Onion services allow people to browse but also to publish anonymously, including publishing anonymous websites.
Onion services are also relied on for metadata-free chat and file sharing, safer interaction between journalists and their sources like with SecureDrop or OnionShare, safer software updates, and more secure ways to reach popular websites like Facebook.
These services use the special-use top level domain (TLD) .onion (instead of .com, .net, .org, etc..) and are only accessible through the Tor network.
When accessing a website that uses an onion service, Tor Browser will show at the URL bar an icon of a little green onion displaying the state of your connection: secure and using an onion service.
And if you're accessing a website with HTTPS and onion service, it will show an icon of a green onion and a padlock.