The arithmetic arts (or sciences) have to do with basic numbers and their manipulation. Useful therefore for merchants of all sorts, and for tax collectors.
Many people would prefer that monarchs don't get too acquainted in these skills or they might ask why the taxes they receive are smaller than the ones collected.
Fundamentals of arithmetic are the disciplines of addition (you had some, now you have more: how much do you have in total ?), subtraction (you want to give away (or sell) some of what you have: how much will you have left ?), multiplication (if a cow bears two calves, and you have twenty cows, how many calves will you get ?), and division (you have an amount of money, and three barbarians (who are definitely not Vikings in any way, shape or form) -- how can you split your money up so that everyone is happy, and how many pieces can they break you into if you do not ?)
Later, under Arabic influences, concepts such as zero, and fractions were introduced.