Django UserAdmin's add_fieldsets?

I'm following a tutorial on making a custom User for authentication purposes. The tutorial used a certain property add_fieldsets within UserAdmin. What does this mean? I can't seem to find any documentation on this.

Here is the snippet:

class UserAdmin(UserAdmin):
"""Define admin model for custom User model with no email field."""

    fieldsets = (
        (None, {'fields': ('email', 'password')}),
        ('Personal info', {'fields': ('first_name', 'last_name')}),
        ('Permissions', {'fields': ('is_active', 'is_staff', 'is_superuser', 'groups', 'user_permissions')}),
        ('Important dates', {'fields': ('last_login', 'date_joined')}),
        ('Contact info', {'fields': ('contact_no',)}),)

    add_fieldsets = (
        (None, {
            'classes': ('wide',),
            'fields': ('email', 'password1', 'password2'),}),)

    list_display = ('email', 'first_name', 'last_name', 'is_staff')
    search_fields = ('email', 'first_name', 'last_name')
    ordering = ('email',)

Here is the tutorial I was following: How to use email as username for Django authentication (removing the username)