In WordPress there is a constant called
WP_DEBUG. It is often set in the
wp-config.php file, which is loaded right at the start of processing. When it’s true then PHP Notices may be displayed to the user. These can cause all sorts of problem.
When false then they’re hidden. In a development environment, if you have a Notice tolerance level of zero then you’ll want to set it to true and hope to never see Notices. In a production environment best practices suggest you set it to false.
See also Debugging in WordPress.