Once everything is ready to finalize the installation your system will need to reboot to begin.
You will now have 30 minutes before your system automatically restarts for that so this is the time to close critical work so you do not lose any work in progress.
If you click/tap Restart Later the upgrade will be completed when the system is not in use.
When you click/tap Restart Later the Update Assistant will minimize to the system tray while it waits for your system to not be in use.
Honestly though, since you voluntarily started this upgrade process just go ahead and click/tap Restart Now and get it started.