🐧 What is different about the gnu c standard used in gnu/linux OS?

Hi I was doing some reading about the C standard stuff, and it was mentioned that gnu c is a different standard as opposed to the regular c stuff and that the whole linux kernel was written the "non c standard". What exactly is different about this compared to the regular c standard, and what does it have to do with Linux/how the kernel was written?

I've been using Linux for awhile but am still completely new to the whole stuff, and have very little experience in programming.

Here is where I read this from and you can find what I am referencing there at the bottom of the first reply, he wrote quite a bit.

https://stackoverflow.com/questions/17206568/what-is-the-difference-between-c-c99-ansi-c-and-gnu-c

I hope this isn't too complicated to explain and don't want to trouble anyone who might have to explain a lot, but I am generally curious.

Thank you!