What are their names?
SELECT t1.store_name FROM store AS t1 JOIN store_district AS t2 ON t1.store_id   =   t2.store_id JOIN district AS t3 ON t2.district_id   =   t3.district_id WHERE t3.district_id  =  15
their names refers to store names (store_name) associated with a specific district (district_id = 15); joining tables (store, store_district, district) refers to relationships among these entities based on store_id and district_id;