Most Pluggable Transports, such as obfs3 and obfs4, rely on the use of “bridge” relays. Like ordinary Tor relays, bridges are run by volunteers; unlike ordinary relays, however, they are not listed publicly, so an adversary cannot identify them easily. Using bridges in combination with pluggable transports helps to disguise the fact that you are using Tor.
Other pluggable transports, like meek, use different anti-censorship techniques that do not rely on bridges. You do not need to obtain bridge addresses in order to use these transports.
Because bridge addresses are not public, you will need to request them yourself. You have two options:
Visit https://bridges.torproject.org/ and follow the instructions, or
Email bridges@torproject.org from a Gmail, Yahoo, or Riseup email address, or
Once you have obtained some bridge addresses, you will need to enter them into Tor Launcher.
Choose “yes” when asked if your Internet Service Provider blocks connections to the Tor network. Select “Use custom bridges” and enter each bridge address on a separate line.
Click “Connect”. Using bridges may slow down the connection compared to using ordinary Tor relays. If the connection fails, the bridges you received may be down. Please use one of the above methods to obtain more bridge addresses, and try again.