Websites ending in .onion are on the Tor network. You need to be connected to and using Tor (The Onion Router, see the other comments) to be able to connect them.
In short, Tor connects you to the internet by splitting bits of your connection across all the other Tor users. This makes it much harder to trace your activity if you are browsing anonymously (use the Tor Browser in its default configuration and window size, DO NOT SIGN INTO ANYTHING).
Websites ending in .onion are on the Tor network. You need to be connected to and using Tor (The Onion Router, see the other comments) to be able to connect them.
In short, Tor connects you to the internet by splitting bits of your connection across all the other Tor users. This makes it much harder to trace your activity if you are browsing anonymously (use the Tor Browser in its default configuration and window size, DO NOT SIGN INTO ANYTHING).