HUSNG refers to a tournament played only between two players with a winner takes all payout structure.
It is an abbreviation of Heads-Up Sit &Go, a format where two players pay an identical buy-in and battle it out until one has all the chips. Since this is a tournament format, players do not have the option to leave earlier, and it must be played will the winner is declared who takes down both buy-ins.
This format can be really fun to play since you are forced to adjust to your opponent and play your best to win against him since you do not have the option of simply folding and waiting for good cards as you can do in a full-ring game. At the same time, it is quite hard to master since you need to understand how to play in various stack depths and quickly change your strategy based on how another player is playing.