What does GRUB_TIMEOUT do?

GRUB_TIMEOUT specifies the duration (in seconds) that the GRUB menu is displayed before automatically booting the default operating system entry. For example, if GRUB_TIMEOUT=5, the menu will show for 5 seconds, allowing the user to select a different entry if desired. If set to 0, the menu will not be displayed at all, and the default entry will boot immediately.