What does ±0.5% FS mean in practice?

FS means full scale: the error is a fixed band across the whole range, not a percentage of the reading.

A 0–1.6 MPa unit with ±0.5% FS is within ±8 kPa anywhere in its range. At a 0.2 MPa working point that is ±4% of the reading, which matters if you are used to read-based accuracy figures.