How Shiro Check User's Password

[Andy]

*hi,I study shiro just now,and read some code like below,but I have one
question:the user's password\role stored in database,how shiro get the
password\role and check it? Thanks.

code:
UsernamePasswordToken token =
        new UsernamePasswordToken(username, password);*
* try {
                Subject subject = SecurityUtils.getSubject();
                subject.login(token);
                token.clear();
                url = "/secure/index.jsp";
        }*