Kng is always used in the music styles hardstyle and hardcore. It is another word for the bass in a track. K is the bass itself and NG is the basssdrum which is esssential in a hardstyle or hardcore track. If there's not a NG present in a track, then it often a crappy song.
Most of the people use UKE instead of KNG.
There are many forms of the bass like KNGOI, KNGAU, KNGIE and so on.
Second thing is FNG, which is almost the same as KNG. With all of the above examples of course.
A simple house or pop track:
k k k k k k k k k k k k k k k
Hardstyle and Hardcore:
Kng kng kng kng kng kng kng kng kng kng kng kng kng