PHP question

They are called function parameters (also called arguments).
https://php.net/manual/en/functions.arguments.php

If you look at the add_action function you’ll see it’s the priority parameter (third):
https://codex.www.ads-software.com/Function_Reference/add_action

add_action( $hook, $function_to_add, $priority, $accepted_args );

It’s used to specify when the callback function ‘reactor_do_nav_bar’ is executed. 10 is the default. Lower numbers correspond with earlier execution, and functions with the same priority are executed in the order in which they were added to the action.

Look at the reactor_option function to know what the parameters are used for.

That’s really helpful.

Thank you.