Dyscalculia is a condition that makes it hard for someone to learn arithmetic. Somebody with dysclaculia may not be able to even understand very basic maths problems that will seem intuitive to everyone else. They may learn correct answers to certian problems by rote but they may not be very confident about them.
Dyscalculia is similar to dyslexia but it is applied to numbers instead of words. But there is also a great overlap between the two conditions.