"You must also have a 2:1 score in..." what does it mean?

I saw this requirement popping up in one of the masters programs I am looking at. I study my bachelor's in HK, and we have a mixed system with GPA and honours classes both listed.

In my university, a 2:1 in terms of major GPA is between 2.85 and 3.59. So it can mean B (3.0) or above?

Yes

Universities Forum Helper

The 2:1 requirement will most likely be in terms of UK degrees. They will assess international degree qualifications against that country's standards - they will usually be familiar with these and may even list these on their website elsewhere (for example they may require a particular GPA or they may simply require the given classification).

Without knowing which UK university it is you're looking at I can't make any other suggestions though.