about abstract user

class User(AbstractUser):      tweet = models.ManyToManyField(Tweet, blank=True)      follower = models.ManyToManyField(settings.AUTH_USER_MODEL, blank=True)      pass    class Tweet(models.Model):      tweet = models.TextField()      favourite = models.ManyToManyField(settings.AUTH_USER_MODEL, blank=True, related_name='user_favourite')        def __unicode__(self):          return self.tweet


1) in above user model has passed AbbstractUser so what does it mean 
2) also in follower field of User model ManyToManyField has setting.AUTH_USER_MODEL what does it mean

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