WordPress at a glance
filter is not described

users_pre_query filter-hook . WP 5.1.0

Filters the users array before the query takes place.

Return a non-null value to bypass WordPress's default user queries. Filtering functions that require pagination information are encouraged to set the total_users property of the WP_User_Query object, passed to the filter by reference. If WP_User_Query does not perform a database query, it will not have enough information to generate these values itself.

Usage

add_filter( 'users_pre_query', 'filter_function_name_6236', 10, 2 );
function filter_function_name_6236( $results, $this ){
	// filter...

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

Where the hook is called

WP_User_Query::query()
users_pre_query
wp-includes/class-wp-user-query.php 610
$this->results = apply_filters_ref_array( 'users_pre_query', array( null, &$this ) );

Where the hook is used (in WP core)

Does not used.