Extracting the Tor Browser Bundle
You downloaded the file to your Downloads directory, which is a subdirectory of your home directory. When you open a terminal, you would not normally be in your Downloads directory. You would normally be in your home directory.
To change directory into your Downloads directory, use the cd command:
cd Downloads
That works if you're in your home directory already, which you probably are when you open a terminal window. However, you can also put ~/ in front of Downloads. The ~/ gets expanded into the full path to your home directory.
So this works from anywhere:
cd ~/Downloads
If you ever want or need to, you can return to your home directory simply with cd, though cd ~ or cd ~/ would also work.
If you actually want to extract the archive in your Downloads folder, you can then do so with the command you ran (assuming the filename is correct for what you downloaded). However, the Tor Browser Bundle doesn't really install--it unpacks to a directory full of files, and you just run it from there. So I suggest unpacking it somewhere more suitable. I would probably put it straight in my home directory:
cd
tar xf Downloads/tor-browser-linux64-7.5.3_en-US.tar.xz
(Future readers will want to use the latest available Tor Browser Bundle, whose filename will eventually be different at least in its version number. It is extremely important to avoid using an old version because security vulnerabilities--including vulnerabilities of the sort that could compromise your privacy--are discovered from time to time, and then new versions are released that fix those bugs. The linux64 part may also be different if you're running the 32-bit version of Ubuntu and thus downloaded the 32-bit version of the Tor browser bundle.)
You don't have to use that exact command. I omitted the v because I didn't feel the need to see a list of all the files being extracted. I omitted the J because, for many years now, tar has automatically detected what type of compression is used.
That extracts the contents of the archive into your home directory. Currently this consists of a single directory called tor-browser_en-US. Inside that directory is a file called start-tor-browser.desktop, which works as a launcher for starting the program graphically, and a subdirectory Browser which contains everything else.
The suggested way to set up the Tor Browser to be run easily is to execute that outer start-tor-browser.desktop file. The rest of this answer is about how to do that as well as what, exactly, that does the first time you do it.
Launching the Tor Browser Bundle for the First Time
After you run that tar command shown above, I recommend that you change into the tor-browser_en-US directory that it created:
cd tor-browser_en-US
And then look at the contents of that outer .desktop file, which explains what to do:
cat start-tor-browser.desktop
Even though I'm giving instructions here, I still recommend you read that file. Part of what that file currently tells you is:
# This file is a self-modifying .desktop file that can be run from the shell.
# It preserves arguments and environment for the start-tor-browser script.
#
# Run './start-tor-browser.desktop --help' to display the full set of options.
#
# When invoked from the shell, this file must always be in a Tor Browser root
# directory. When run from the file manager or desktop GUI, it is relocatable.
#
# After first invocation, it will update itself with the absolute path to the
# current TBB location, to support relocation of this .desktop file for GUI
# invocation. You can also add Tor Browser to your desktop's application menu
# by running './start-tor-browser.desktop --register-app'
So I suggest you run:
./start-tor-browser.desktop
Then you can copy that outer .desktop file wherever you like and it will still work as a launcher for the Tor Browser.
You may also, or alternatively, want to put it in your desktop environment's menus. As explained in the excerpted text above, running this command (still from within the extracted tor-browser_en-US directory) should do that:
./start-tor-browser.desktop --register-app
