join()方法的wait为什么会使调用者wait

public final synchronized void join(long millis)throws InterruptedException 
if (millis == 0) {
    while (isAlive()) {
        wait(0);
}
//在main里面调用thread.join(),isAlive判断thread是否存货我能理解,可接下来的wait为什么是让main等待了