No, your PATH variable determines where your shell looks for commands when you enter them. It usually includes directories like /usr/bin and /usr/local/bin, where the operating system commands are stored. If you want to be able to use the 'fetch' command from any directory, then the directory where the 'fetch' command is stored (your depot_tools checkout directory) needs to be in your PATH. Then you can run `fetch android` from the directory where you want to sync the source.
There are lots of resources online about setting your PATH, and files like .bashrc -- you should probably do some research there if you still need to learn more. chromium-dev isn't the right forum for more general Ubuntu advice, but hopefully you're at least pointed in the right direction :)