Math Unicorns are Unicorns which have special magical abilities to do math. As an experienced calculus student would know, calculus is about 75% magic. Using the magic spells of the unicorn horn, He is able to harness the power of that magic.
Math Unicorns are referenced by Twilight Sparkle in Boast Busters:
"True, for ponies whose talents are for things like cooking or singing or math, but what if a unicorn's special talent is magic?" It is certainly true that one could benefit from a few magic spells when doing math.
Especially that trick where you write stuff by kinetisis with a writing implement.
Math Unicorns are very important in the engineering and astronomical parts of life in Ponyville.
They make all the calculations for where the heavenly bodies must be positioned.
They also are just acedemics, as well as professors occasionally
They do accounting quite easily through the use of automated spells.
They also take the Census by counting everything extremely quickly and accurately.