The phrase "reboot button" refers to starting something over or giving it a fresh start, similar to how you can restart a computer with a reboot button.Full definition
The big red giant evil Reboot Button is Back It's Latest victim.... (eliteforcepodcast.com)
APM + is a simple way to customize Android's power menu, not only bringing useful tweaks like an instant reboot button to the forefront, but also adding the ability to throw toggles — like Wi - Fi and flashlight — into the menu. (howtogeek.com)
If an existing module is updated, the notification provides Reboot and Soft Reboot buttons to let the changes take effect. (android.gadgethacks.com)