Similar to when you can't read, only this means mathematically challenged. Math was most likely your worst subject because you just don't know what you're doing in that class. Math really stumps you if you're math-illiterate; you often fail to comprehend numbers, adding, subtracting, etc.
Complete opposite of math-literate.
Marilyn is really excellent at english and history and always helps her friends with those subjects, but when it comes to geometry she's really math-illiterate.