WordPress at a glance
filter is not described

posts_pre_query filter-hook . WP 4.6.0

Filters the posts array before the query takes place.

Return a non-null value to bypass WordPress's default post queries.

Filtering functions that require pagination information are encouraged to set the found_posts and max_num_pages properties of the WP_Query object, passed to the filter by reference. If WP_Query does not perform a database query, it will not have enough information to generate these values itself.

Usage

add_filter( 'posts_pre_query', 'filter_function_name_1407', 10, 2 );
function filter_function_name_1407( $posts, $this ){
	// filter...

	return $posts;
}
$posts(array/null)
Return an array of post data to short-circuit WP's query, or null to allow WP to run its normal queries.
$this(WP_Query)
The WP_Query instance (passed by reference).

Where the hook is called

WP_Query::get_posts()
wp-includes/class-wp-query.php 2756
$this->posts = apply_filters_ref_array( 'posts_pre_query', array( null, &$this ) );

Where the hook is used (in WP core)

Does not used.