It's just a measurement of the average red blood cell volume (MCV stands for Mean Corpuscular Volume).
An MCV test comes as pretty standard with any blood test.
They way doctors work out your MCV is by taking a blood sample, compacting the red blood cells into one block (by spinning them around in a centrifuge).
Then they measure the volume of this block, divide it by the individual number of blood cells, and then multiply it by 10.
This will give you an MCV reading, and can be used in the diagnosis of conditions such as anemia, and related health problems.