What does sudo apt-get autoremove do ?

sudo apt-get autoremove

Whenever you install an application (using apt-get) the system will also install the software that this application depends on. It is common in Ubuntu/Linux that applications share the same libraries. When you remove the appplication the dependency will stay on your system.
So apt-get autoremove will remove those dependencies that were installed with applications and that are no longer used by anything else on the system.