Most Pluggable Transports, such as 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 conceal the fact that you are using Tor, but may slow down the connection compared to using ordinary Tor relays.
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.
GETTING BRIDGE ADDRESSES
Because bridge addresses are not public, you will need to request them yourself. You have a few options:
USING MOAT
If you're starting Tor Browser for the first time, click "Tor Network Settings" to open the Tor settings window.
Under the "Bridges" section, select the checkbox "Use a bridge", then choose "Request a bridge from torproject.org" and click "Request a Bridge..." for BridgeDB to provide a bridge.
Complete the Captcha and click "Submit". Click "Connect" to save your settings.
Or, if you have Tor Browser running, click on "Preferences" (or "Options" on Windows) in the hamburger menu (≡) and then on "Tor" in the sidebar.
In the "Bridges" section, select the checkbox "Use a bridge", and from the option "Request a bridge from torproject.org", click "Request a New Bridge..." for BridgeDB to provide a bridge.
Complete the Captcha and click "Submit". Your setting will automatically be saved once you close the tab.
ENTERING BRIDGE ADDRESSES
If you're starting Tor Browser for the first time, click "Tor Network Settings" to open the Tor settings window.
Under the "Bridges" section, select the checkbox "Use a bridge", choose "Provide a bridge I know" and enter each bridge address on a separate line.
Click "Connect" to save your settings.
Or, if you have Tor Browser running, click on "Preferences" (or "Options" on Windows) in the hamburger menu (≡) and then on "Tor" in the sidebar.
In the "Bridges" section, select the checkbox "Use a bridge", and from the option "Provide a bridge I know", enter each bridge address on a separate line.
Your settings will automatically be saved once you close the tab.
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.