second parameter of admin django code example

Example 1: show post id on django admin interface

class BookAdmin(admin.ModelAdmin):
    readonly_fields = ('id',)

admin.site.register(Book, BookAdmin)

Example 2: what is admin.tabularinline django

from django.contrib import admin

class BookInline(admin.TabularInline):
    model = Book

class AuthorAdmin(admin.ModelAdmin):
    inlines = [
        BookInline,
    ]