FS = FULL SCALE = maximum reading.
It means that the accuracy is such that the reading is probably within + or - 0.5% of the FULL SCALE reading.
This is a very important and easily overlooked qualification of the result.
If I have a reading of 1 Volt and the accuracy is +/- 0.5% it means that the actual result should lie in the range 1 - 0.5% x 1 to 1 + 0.5% of 1
= 0.995V to 1.005 V
However - if I measure the result on the 10V range then 0.5% of 10V = 0.5% of Full Scale
So 1V +/- 0.5% of FS
= 0.95V to 1.05V.
On the 100V range, 1V +/- 0.5% FS lies in the range
0.5V to 1.50 V. !!!!
The reason for specifying results in this manner is that the error experienced on a given range tends to largely be constant regardless of the actual reading. So, as the input gets smaller the error becomes increasingly large in proportion.
So eg on a 100V range a reading of 0.5V +/- 0.5% FS lies in the range
0 to 1V !