MYSQL primary key use btree 是什么含义了解一下

CREATE TABLE `sth_definition` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `analyseId` bigint(20) DEFAULT NULL,
  `levelCondition` varchar(2048) DEFAULT NULL,
  `filed1` double DEFAULT NULL,
  `dfdf` double DEFAULT NULL,
  `dfsdf` double DEFAULT NULL,
  `dfdf` varchar(255) DEFAULT NULL,
  `sf` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE,
  KEY `id` (`id`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=7974 DEFAULT CHARSET=utf8

primary key use btree 是什么含义了解一下