A new version of the Linux operating system kernel. The kernel is what all Linux-based operating systems have in common. You can get it for free.
You usually get it in its source code (as written by Linus Torvalds, Alan Cox, etc.), but in order for the operating system to boot, the kernel must be in machine code, ie. you must convert the kernel from source to machine code, a process called compiling.
In the compilation process, you can set certain parameters and only include modules for the hardware you plan to use.
Compiling a kernel is rarely needed and mainly on server machines.
Compiled kernels packaged for the respective package system and processor architecture are available for almost all Linux distributions. Hardware modules are added to them, which differ depending on the distribution and the philosophy of the developers.