A basic cat flap is just a hole you can insert into your door to allow your cat to come and leave as they please. This design isn't extremely secure and could give access to burglars. You might also want to look into upgrading to an automated or lockable model https://cat-flap-installation-nea60776.amoblog.com/adding-a-cat-flap-to-your-bifold-doors-52603086