KaiOS is a Linux Operating System based on the Android Operating System, and is developed specifically as the operating system for use with feature phones (which are often flip phones, and also are keypad based phones). As an operating system, it handles the basic functionality of apps or programs on these phones.
Despite being somewhat unwieldy by the standards of modern smartphones, a feature phone still has a number of different programs, including a camera (with video), text messaging, a web browser, email, as well as sometimes other things such as the YouTube app. All of these require some type of operating system to handle input/output and the file system, and a lightweight form of Linux is ideally suited for the task. However, from the user side, there is very little obvious sign that this is Linux---there is no command line that you can try to access and use through the 12 key keyboard. The main sign that this is Linux is in its extreme stability and low power consumption. I have had my Alcatel flip phone since 2000, and it has never had any type of glitch or lag or other problems that might occur with consumer electronic OSs. So the KaiOS is just a very compact, stable operating system that is perfect for running the few programs that might occur on a feature phone.
The KaiOS is developed by the KaiOS company, which despite having a "mission statement" about digital inclusion and the like, seems to be a for-profit company, owned by a Chinese telecom firm.
https://www.kaiostech.com/