A bridge is
|“||[t]he interconnection between 2 networks using the same communications method, the same kind of transmission medium, and the same addressing structure; also the equipment used in such an interconnection.||”|
A bridge program is a computer program written for a specific pair of software applications which exchange data. This bridge program converts the data (either in-coming or out-going) from one format to another so the two software applications can work together without having to be modified or rewritten.
"Bridges are Tor relays which are not publicly listed as part of the Tor network. Bridges are essential censorship-circumvention tools in countries that regularly block the IP addresses of all publicly listed Tor relays, such as China. A bridge is generally safe to run in your home, in conjunction with other services, or on a computer with your personal files."