A geometric modeling kernel is like a magic factory that helps people make very precise shapes using computers. Think of playing with blocks, except instead of stacking blocks together, you use really fancy math to make 3D shapes on a computer.
The kernel helps people make these shapes by giving them tools to create lines, curves, planes, and other pieces that fit together perfectly. The kernel also helps people move and change these shapes in specific ways that they want.
Just like how you can mix and match different colored blocks to make cool designs, people can use the geometric modeling kernel to make complex parts and assemblies on the computer. These designs can be saved and sent to machines that can make them in real life, like 3D printers or CNC machines.
Without the kernel, it would be very difficult to make precise shapes on the computer, because there would be no magic factory to help people. So, the kernel is an important tool for people who want to make very accurate parts and designs using a computer.