What is related_name used for?

The related_name attribute specifies the name of the reverse relation from the User model back to your model.

If you don't specify a related_name, Django automatically creates one using the name of your model with the suffix _set, for instance User.map_set.all().

If you do specify, e.g. related_name=maps on the User model, User.map_set will still work, but the User.maps. syntax is obviously a bit cleaner and less clunky; so for example, if you had a user object current_user, you could use current_user.maps.all() to get all instances of your Map model that have a relation to current_user.

The Django documentation has more details.

To add to existing answer - related name is a must in case there 2 FKs in the model that point to the same table. For example in case of Bill of material

@with_author class BOM(models.Model):     name = models.CharField(max_length=200,null=True, blank=True)    description = models.TextField(null=True, blank=True)    tomaterial =  models.ForeignKey(Material, related_name = 'tomaterial')    frommaterial =  models.ForeignKey(Material, related_name = 'frommaterial')    creation_time = models.DateTimeField(auto_now_add=True, blank=True)    quantity = models.DecimalField(max_digits=19, decimal_places=10)

So when you will have to access this data you only can use related name

 bom = material.tomaterial.all().order_by('-creation_time')

It is not working otherwise (at least I was not able to skip the usage of related name in case of 2 FK's to the same table.)

The related_name argument is also useful if you have more complex related class names. For example, if you have a foreign key relationship:

class UserMapDataFrame(models.Model):    user = models.ForeignKey(User) 

In order to access UserMapDataFrame objects from the related User, the default call would be User.usermapdataframe_set.all(), which it is quite difficult to read.

Using the related_name allows you to specify a simpler or more legible name to get the reverse relation. In this case, if you specify user = models.ForeignKey(User, related_name='map_data'), the call would then be User.map_data.all().