This word is used when you are heavily confused. You can't say it on command, it just comes out of your mouth when your brain doesn't know what the fuck is going on.
"Ok I'm on question 14 in the exam, HUH, what on earth is this question???"
Boy1: "Do you know how to fix this problem on my laptop bro?"
Boy2: "HUH, what did you do to your laptop!"