Android phones have a recovery mode that lets you access the bootloader and wipe data if you're unable to normally boot your phone.

If your Android device is stuck in Recovery mode, you can perform a hardware master reset to get out of the Recovery menu loop.

Once you’re in the recovery menu, use the Volume keys to navigate through the options and use the Power button to select an option.

Still, the stock recovery in Android M is a little more capable than before. There appear to be three new options in the recovery menu: apply update from SD card, mount /system, and view recovery ...