Adjective. The term is used to indicate that someone or something has a nerd
-like quality. It is also used by geeks to describe other, more hard-core
geeks. It's connotation may be positive or negative depending on tone.
The term comes from the software industry, where kernel programming is among the most technically involved. From an informal standpoint, the more connected people are with kernel software, the higher the chances are of them being geek-like.
John is always talking about role-playing games on his computer - he's close to the kernel.