WordPress at a glance
function is not described

WP_Rewrite::add_rule() public WP 2.1.0

Adds a rewrite rule that transforms a URL structure to a set of query vars.

Any value in the $after parameter that isn't 'bottom' will result in the rule being placed at the top of the rewrite rules.

It's a method of the class: WP_Rewrite{}

No Hooks.

Return

Null. Nothing.

Usage

$WP_Rewrite = new WP_Rewrite();
$WP_Rewrite->add_rule( $regex, $query, $after );
$regex(string) (required)
Regular expression to match request against.
$query(string/array) (required)
The corresponding query vars for this rewrite rule.
$after(string)
Priority of the new rule. Accepts 'top' or 'bottom'.
Default: 'bottom'

Changelog

Since 2.1.0 Introduced.
Since 4.4.0 Array support was added to the $query parameter.

Code of WP_Rewrite::add_rule() WP 5.5.1

<?php
public function add_rule( $regex, $query, $after = 'bottom' ) {
	if ( is_array( $query ) ) {
		$external = false;
		$query    = add_query_arg( $query, 'index.php' );
	} else {
		$index = false === strpos( $query, '?' ) ? strlen( $query ) : strpos( $query, '?' );
		$front = substr( $query, 0, $index );

		$external = $front != $this->index;
	}

	// "external" = it doesn't correspond to index.php.
	if ( $external ) {
		$this->add_external_rule( $regex, $query );
	} else {
		if ( 'bottom' === $after ) {
			$this->extra_rules = array_merge( $this->extra_rules, array( $regex => $query ) );
		} else {
			$this->extra_rules_top = array_merge( $this->extra_rules_top, array( $regex => $query ) );
		}
	}
}