Can anyone explain to me...

Goods and Services Tax. It's a general tax added to a range of G&S as set out in the legislation. In some countries it affects just about everything, in others it excludes 'essentials', and in others it is even more selective. Often it would come hand-in-hand with a reduction in personal income tax, so that it becomes more of a 'user pays' system. To the Mr Average (local) in HK the 'damage' will depend on how much of a consumer he is. I think that Mr Average probably doesn't have the lavish lifestyle that would result in too much of a GST impact, as basic foodstuffs and rentals (at least below a certain level) are likely to be exempt. But things like cars, petrol, cinema tickets, restaurants, hotels etc etc are likely to be included. I haven't actually seen the list.

Whoops. Just as I hit the [Reply] button I noticed the 'expat' qualifier. He is more likely to be hit in the pocket than the local Mr Average due to the different lifestyle and spending patterns. Overall impact could depend on whether he pays HK income tax only (in which case there's likely to be a bit of a trade-off there) or tax in his home country (in which case the GST is just an additional expense).