Arithmetic is another word for math, specifically the areas of math having to do with numbers and calculating.
If you're good at adding, subtracting, dividing, and multiplying, then you're good at arithmetic, which is a big part of math. Arithmetic has to do with doing calculations. To answer most math problems, including almost all word problems, you need arithmetic. The most basic arithmetic is 2 plus 2, but as anyone who's ever taken algebra can tell you, it can get a lot more complicated than that.