what is the purpose of this $table->primary(['user id', 'class id']) in laravel code example

Example 1: laravel 6 auth

composer require laravel/ui "^1.0" --dev

php artisan ui vue --auth

Example 2: update query laravel

$update = \DB::table('student') ->where('id', $data['id']) ->limit(1) ->update( [ 'name' => $data['name'], 'address' => $data['address'], 'email' => $data['email'], 'contactno' => $data['contactno'] ]);