SQL语句中的疑问？

KEY `fk_award_shop_idx` (`shop_id`),这句是什么意思？作用是什么

DROP TABLE IF EXISTS `tb_award`;

/*!40101 SET @saved_cs_client     = @@character_set_client */;

 SET character_set_client = utf8mb4 ;

CREATE TABLE `tb_award` (

  `award_id` int(10) NOT NULL AUTO_INCREMENT,

  `award_name` varchar(256) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,

  `award_desc` varchar(1024) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,

  `award_img` varchar(1024) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,

  `point` int(10) NOT NULL DEFAULT '0',

  `priority` int(2) DEFAULT NULL,

  `create_time` datetime DEFAULT NULL,

  `last_edit_time` datetime DEFAULT NULL,

  `enable_status` int(2) NOT NULL DEFAULT '0',

  `shop_id` int(10) DEFAULT NULL,

  PRIMARY KEY (`award_id`),

  KEY `fk_award_shop_idx` (`shop_id`),

  CONSTRAINT `fk_award_shop` FOREIGN KEY (`shop_id`) REFERENCES `tb_shop` (`shop_id`)

) ENGINE=InnoDB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8;

/*!40101 SET character_set_client = @saved_cs_client */;