Just that the salary would be 24k for full time hours and that the equivalent rate applies according to the hours worked; ie if your p/t hours are half the number that full timers do, you would be paid half the salary (12k).
"it's a 16 hour a week job would I get the 24k? or less."
You'd get less. What it means is that if you worked full time you'd get 24k. How much you'd get depend on what the standard working week is at that firm.
So, if they normally work 40 hours, you'll get 24000/40 x 16 = 9,600
In order to calculate what you would be paid per annum for 16 hrs pro-rata you need to know how many hours the full time staff work, could be 37.5, 39, 40, etc. Didn't they give you an hourly rate? Just ask them.
So you need to find out how many hours equates to full time or ask what fraction/percentage/decimal this post is of a full time post. A full time post is likely to be 35-40 hours so the actual pay for 16 hours is likely to be nearer £10k