When you start Tor Browser, you will see the Connect to Tor window.
If you are on a relatively fast connection, but this bar seems to get stuck at a certain point, try the 'Connection Assist' or see the Troubleshooting page for help solving the problem.
If Tor is blocked in your location, trying a bridge may help. Connection Assist can choose one for you using your location.
If Connection Assist is unable to determine your location or you want to configure your connection manually instead, you can select your region from the dropdown menu and click on 'Try a Bridge'.
The Connection Assist informs you about the state of your Internet connection and your connection to the Tor network.
The first checkbox is 'Quickstart'. If selected, every time you open Tor Browser, it will try to connect with your previous network settings.
If you know your connection is censored, or you have tried and failed to connect to the Tor network and no other solutions have worked, you can configure Tor Browser to use a pluggable transport.
'Bridges' will display the Circumvention section to configure a pluggable transport or to connect using Bridges.
If your connection uses a proxy you can configure it by clicking on 'Settings ...' against 'Configure how Tor Browser connects to the Internet'.
In most cases, this is not necessary. You will usually know if you need to select this checkbox because the same settings will be used for other browsers on your system.