Each time WordPress loads the page, it will run main loop.
The loop is the way to iterate over all elements related to the page you are currently on.
Main loop will work on a global
WP_Query object. The query has a globalized method
have_posts(), that allows us to loop through all results. Finally inside the loop you can call
the_post() method (also as a global function), which sets global post object to the current post inside the loop, and sets the postdata to the current post. Thanks to this you can call functions like
the_author (template tags) directly inside the loop.
For example if you are on posts lists, main loop will contain a query object with all posts.
If you are on single post (or page), it will contain a query with single post (page) you are currently on.
if ( have_posts() ) : while ( have_posts() ) : the_post(); var_dump( $post ); endwhile; endif;