FlaskのUserMixinとは何ですか？質問する

from datetime import datetime
from werkzeug.security import generate_password_hash
from werkzeug.security import check_password_hash
from flask_login import UserMixin
from app import db


class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True)
    email = db.Column(db.String(64), index=True, unique=True)
    password_hash = db.Column(db.String(64))
    posts = db.relationship('Post', backref='author', lazy='dynamic')

    def set_password(self, password):
        self.password_hash = generate_password_hash(password)

    def check_password(self, password):
        return check_password_hash(self.password_hash, password)

    def __repr__(self):
        return '<User{}>'.format(self.username)