When using Tor with Tails in its default configuration, anyone who can observe the traffic of your Internet connection (for example your Internet Service Provider and perhaps your government and law enforcement agencies) can know that you are using Tor.
This may be an issue if you are in a country where the following applies:
Using Tor is blocked by censorship: since all connections to the Internet are forced to go through Tor, this would render Tails useless for everything except for working offline on documents, etc.
Using Tor is dangerous or considered suspicious: in this case starting Tails in its default configuration might get you into serious trouble.
Tor bridges, also called Tor bridge relays, are alternative entry points to the Tor network that are not all listed publicly. Using a bridge makes it harder, but not impossible, for your Internet Service Provider to know that you are using Tor.
If you are in one of the situations described above you might want to use Tor bridges in Tails. Please also read The Tor Project's dedicated page about bridges to get a general idea about what bridges are.
In order to use bridges, you must know in advance the address of at least one bridge. The Tor Project distributes bridge addresses in several ways, for example from their website and via email.
Tails provides a bridge mode that guides you through the process of setting up bridges before connecting to Tor.
Note that you must have at hand at least one bridge address before starting Tails. For example, you can write it down on a piece of paper or store it in a persistent volume.
To activate the bridge mode, add the bridge boot option to the boot menu. For detailed instructions, see the documentation on using the boot menu.
The Tor Project's documentation on bridges mainly focuses on censorship circumvention, this means when the usage of Tor is blocked by censorship. If using Tor is dangerous or considered suspicious in your country, then there are some extra rules that you should follow in order to prevent you from being identified as a Tor user.
Always start Tails in bridge mode.
Only use obfuscated bridges since they are harder to identify than other bridges.
The less publicly known the bridges are, the better. Unfortunately, since some bridge addresses can be obtained by anyone from the Tor website or by email, it is also possible for an adversary to get the same bridge information by the same means. The Tor Project has some protection against that, but they are far from being perfect.
So the best is if you can find a trusted friend or an organisation in a different country who runs a "private" obfuscated bridge for you. In this case "private" means that the bridge is configured with the option
PublishServerDescriptor 0. Without this option The Tor Project can learn about the bridge and may distribute its address to others and so it could end up in the hands of your adversary.