Mk is the slang used for saying Okay or "you okay" in a question. So say you asked someone to pick up onions, you would say "Pick up some onions, mk?"
You pronounce and say it the way that the M is not pronounced like
the alphabet, but the way it would be pronounced in a word, for example the M at the beginning of Megan. The K is pronounced
the alphabet way, however, i.e. not
in the way you would say "kite". It also can be used to ask someone to do something, as in "Pick up Tommy, mk?" Instead of "Can you pick up Tommy?"