A basic cat flap is simply a hole that you can insert into your door so that your cat can come and leave whenever they want. This design isn't particularly secure and allows burglars access to your home. You may also look into upgrading to an automated or lockable model https://cat-flap-in-upvc-door-pan78996.webbuzzfeed.com/31330970/using-a-cat-flap-in-bifold-doors