Tor: What is an onion service?

Short answer

An onion service is a website or other service reached through a .onion address inside the Tor network. Its server location is hidden from visitors, and the Tor connection to it is end-to-end encrypted without an exit relay. You still need the correct address: Tor can authenticate the address you entered, but it cannot tell you whether a stranger's lookalike address belongs to the organization you expected.

Sources

Tor Project: Onion services.