MCHCMean corpuscular haemoglobin concentration
How concentrated the haemoglobin is inside red cells, rather than how much there is.
Mean corpuscular haemoglobin concentration describes how tightly packed the haemoglobin is within the volume of the cells, rather than the absolute amount per cell that MCH reports.
It matters because concentration and quantity can move independently. A cell can be small and hold little haemoglobin while still being normally concentrated, which helps separate the different causes of anaemia.
In practice MCHC is also a quality check on the sample itself: an unusually high value is more often a laboratory artefact, from a lipaemic sample or cells breaking down in the tube, than a real finding.