To understand this request, it helps to break down the terms:
Because it doesn't "install" to the Program Files directory, you usually don't need a password to run it.
Now go water those parsnips—even if you're supposed to be paying attention in history class.
The mobile version is a great way to play Stardew Valley on-the-go, and it's not blocked by school or work firewalls.
By default, Stardew Valley saves to your %AppData% folder on the C: drive. To make it truly portable, you can use a "batch file" or a portable launcher (like those found on PortableApps) to tell the game to save directly to the USB.