Bridge servers (or “bridges” for short) are Tor servers that aren’t listed in the main Tor directory. Since there is no complete public list of them, even if your ISP is filtering connections to all the known Tor servers, they probably won’t be able to block all the bridges. If you suspect your access to the Tor network is being blocked, you may want to use bridges.
The addition of bridges to Tor is a step forward in the blocking resistance race. It is perfectly possible that even if your ISP filters the Internet, you do not require a bridge to use Tor. So you should try to use Tor without bridges first since it might work.
To use bridges, you have two options. InviZible Pro provides some bridges by default. You can enable these easily. Unfortunately, because these bridges are publically distributed, it is easy for censors to block some of them, so some of them may not work. In this case, you’ll need to get new bridges.
In any case, it is better to request new bridges, because the default ones may stop working over time.
Obfs4 bridges are the most commonly used type of bridges. But depending on your location, ISP or network, you may select the other type.
Meek_lite bridge should be used in situations where the obfs4 bridges are not working or to request new obfs4 bridges.
Snowflake bridge is a new and progressive technology that circumvents censorship. But it can be unstable.
You can change the STUN server for the snowflake bridge if the default server is blocked. One alternative server is stun.ekiga.net
Open MENU -> Tor Settings -> STUN server for SnowFlake Bridges
If you are using InviZible Pro in Root Mode with DNSCrypt and Tor running, make sure that the “*stun” line is disabled in the DNSCrypt blacklist. Otherwise, snowflake bridge will not work.