Tor: What is a Tor bridge?

Short answer

A bridge is a Tor entry relay that is not listed in the ordinary public relay directory. It helps people connect when direct Tor traffic is blocked or when they do not want the local network to easily recognize a public Tor relay connection. Bridges are entry points, not exit relays, and they do not by themselves make a website visit anonymous if you sign in.

Sources

Tor Project: Using bridges, Tor Project: Relay types.