The first number is the diameter of the piece that is threaded. The second number is the pitch.....how many threads per inch there are.
So 1/2"-28 means it is a 1/2 in diameter male thread on the barrel and also a 1/2" female thread on the accessory........ and the 28 means there are 28 threads per inch.
This is a relatively fine thread. If it was say a 1/2"-13....making it up here, I am a metric guy......but if it was a 1/2-13..that's only 13 threads per inch, so the lands and grooves would be much coarser......whereas the 28 per inch pitch, is 28 of them squeezed into the same 1"....much finer.
Generally you will find fine threads wherever precision and high quality is involved. Often on plumbing and construction type fasteners you will find coarser threads.
Coarse threads are easy to cross thread...to strip, compared to fine. But care should be involved any time you attach a fastener. Not to mention that stripping the female thread on an $80(?) or so item like a nice flash hider, and boogering the male barrel thread would be a PITA to correct.
Probably hard to do with these items being hardened and heat treated etc....but still. Always pays to work carefully.