1An onion service
Tor Watch runs as an onion service: a site that lives inside the Tor network instead of on the regular web. Both you and the site stay behind several relays.
2The address
Instead of a name like example.com, the tor watch onion address is a long string ending in .onion. It is generated from the service's cryptographic key.
3Why only Tor Browser
Regular browsers cannot route traffic through Tor, so the torwatch URL simply fails to load in them.
4What that means for you
You need Tor Browser, a correct address and a bit of patience, because pages travel through several relays.