There are many options to enable maintenance mode in WordPress. It can be enabled by WordPress built in maintenance mode options or by using a plugin.
While updating WordPress , theme or plugin WordPress creates a .maintenance file in website root directory.This prevents site visitors from seeing any error during update process.
Create a .maintenance file in website root if want to enable default maintenance mode message
Briefly unavailable for scheduled maintenance. Check back in a minute.
Create a maintenance.php file in wp-content directory for custom maintenance mode message.