When you interpret something, you make sense of it. You could interpret a graph, a foreign language, or even Mona Lisa's odd smile.
If you're ordering food in a foreign land, you may need someone to interpret the menu for you. When you get tripped up or struggle to understand a subject even in your own language — like calculus, for example — sometimes you can find meaning, or interpret it for yourself. Or sometimes you'll need a teacher to work on the problem with you, showing how to interpret the mathematical language.