"Cat Flap" is a term used to describe a small portal or door installed in a larger door or wall to allow cats to enter and exit a house at their leisure. It's a legitimate product and concept. There are various types of cat flaps available in the market, including microchip cat flaps. These devices register your cat's unique microchip number and will only open for them, preventing unwanted strays from entering your home. Some popular brands include SureFlap Microchip Cat Door, PetSafe 4-Way Locking Microchip Entry Cat Door, and Cat Mate Elite Microchip Cat Flap with Timer Control. These products have been tested and reviewed by various sources, indicating their legitimacy. However, as with any product, it's important to research and choose the one that best fits your and your cat's needs.