WordPress at a glance
function is not described

WP_Query::is_category() WP 3.1.0

Is the query for an existing category archive page?

If the $category parameter is specified, this function will additionally check if the query is for one of the categories specified.

This is a method of the class: WP_Query

No Hooks.

Return

true/false.

Usage

$WP_Query = new WP_Query();
$var = $WP_Query->is_category( $category );
$category(mixed)
Category ID, name, slug, or array of Category IDs, names, and slugs.
Default: ''

Code of WP Query::is category: wp-includes/class-wp-query.php WP 5.2

<?php
public function is_category( $category = '' ) {
	if ( ! $this->is_category ) {
		return false;
	}

	if ( empty( $category ) ) {
		return true;
	}

	$cat_obj = $this->get_queried_object();

	$category = array_map( 'strval', (array) $category );

	if ( in_array( (string) $cat_obj->term_id, $category ) ) {
		return true;
	} elseif ( in_array( $cat_obj->name, $category ) ) {
		return true;
	} elseif ( in_array( $cat_obj->slug, $category ) ) {
		return true;
	}

	return false;
}