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Tor is a network of relays run by volunteers all over the world that allow you to bounce your communications on them which allows you to hide the websites you visit from people watching your internet connection, it also prevents websites you visit from learning your physical location.
Because you are bouncing your traffic through relays run by volunteers all over the world and your connection will be affected by bottlenecks and network latency.
Once you open the Orbot, press the big button, and after a few seconds establishing a connection to Tor you will see a message that says 100% connected, and the button will turn green. If you are using the VPN to route a browser’s traffic through Tor, you can also check your connection at any moment going to https://check.torproject.org/, a link created by the Tor team to tell you whether you are connected to Tor or not.
Bridges are Tor relays that help circumvent censorship. You can try bridges if Tor is blocked by your ISP.