"Tor log" is an automatically-generated list of Tor's activity that can help diagnose problems.
When something goes wrong with Tor, you may see an option with the error message to "copy Tor log to clipboard".
You should see an option to copy the log to your clipboard, which you can then paste to a document to show whoever is helping you troubleshoot.
If you don't see this option and you have Tor Browser open, you can navigate to the hamburger menu ("≡"), then click on "Settings", and finally on "Connection" in the side bar.
At the bottom of the page, next to the "View the Tor logs" text, click the button "View Logs...".