Only that it is "mean corpuscular volume."

Among other sites:
http://www.healthcentral.com/mhc/top/003648.cfm
See also section III at:
http://www.lloyd1.com/uthman/nutritional_anemia/nutritional_anemia.html
as well as Part 2 of that series.

MCV is one of the index tests of red blood cell counts.  A high
MCV means you are "macrocytic", (which means your red blood cells
are larger than they should be).  Besides folic acid deficiency,
the other major cause for this would be vitamin B12 deficiency.
Of course, there are other, rarer, causes, too.  If you are
taking MTX, which acts against folic acid, then maybe that is why.
Usually you need to take 400mcg or more when you are on MTX.  If
you eat plenty of veggies and fresh fruit, and are NOT taking
MTX, you shouldn't need more than 400mcg folic acid a day (the
daily value is 200mcg).  There are few side effects of larger doses
of folic acid, HOWEVER, it *can* mask vitamin B12 deficiency (esp
if all they look at is MCV).  But that doesn't mean compensating
by taking more B12.  You may not actually be deficient in B12
(unless you are a strict vegan), but actually have impaired
absorption due to pernicious anemia (lack of a helper called
intrinsic factor) and is common in elderly patients and those
that have lower than normal digestive enzymes.  You can take
tons, but if it isn't going to be absorbed, what good is that?
A funfact that I like about B12 is that the bacteria in our guts
actually cause us to poop out more B12 than we consume, which
demonstrates that B12 deficiency is rarely due to a lack of
availability.  B12 only comes from animal sources, which is why
I said strict vegans can be deficient by diet.