Some Internet Service Providers (ISPs) attempt to prevent users from accessing the Tor network by blocking connections to known Tor relays. Bridge relays (or bridges for short) are relays that help these censored users access the Tor network. Unlike other Tor relays, bridges are not listed in the same public directories as normal relays. Since there is no complete public list of them, even if your ISP is filtering connections to all the known Tor relays, they probably won't be able to block all the bridges.
There are two main ways to learn about a bridge address:
To use private bridges, ask your friends to run Vidalia and Tor in an uncensored area of the Internet, and then click on Help censored users in Vidalia's Relay settings page. Then they should privately send you the Bridge address line at the bottom of their Relay page. Unlike running an exit relay, running a bridge relay just passes data to and from the Tor network, so it shouldn't expose the operator to any abuse complaints.
You can find public bridge addresses by visiting https://bridges.torproject.org. The answers you get from that page will change every few days, so check back periodically if you need more bridge addresses. Another way to find public bridge addresses is to send mail to bridges@torproject.org with the line get bridges by itself in the body of the mail. However, so we can make it harder for an attacker to learn lots of bridge addresses, you must send this request from a Gmail account.
Configuring more than one bridge address will make your Tor connection more stable, in case some of the bridges become unreachable.