A kernel is the central part of an operating system. It acts as a bridge between the programmes you run and the physical components of your computer, such as the processor, memory, and storage devices.
Think of it as a traffic controller. When you open an app, the kernel decides how much processor time it gets, which parts of memory it can access, and whether it can communicate with your hard drive or graphics card. Without the kernel, software and hardware would not be able to work together.
Different operating systems use different kernels. Windows uses its own kernel, macOS uses XNU, and Linux systems use the Linux kernel (which is free and open-source). Each kernel is optimised for its own operating system and the hardware it runs on.
You rarely interact with the kernel directly. Instead, it works invisibly in the background, handling thousands of tasks every second. However, kernel updates are important: manufacturers release them to fix security vulnerabilities, improve performance, or add support for new hardware. On your smartphone or computer, you have probably seen a system update that includes kernel changes.
When choosing a device, you do not need to compare kernels like you would compare processor speed. However, understanding that the kernel exists helps you appreciate why system updates matter and why manufacturers need time to optimise new operating systems for their devices.