Dyscalculia is a difficulty in understanding number related or maths concepts and symbols. This manifests itself in various ways examples of which are, understanding for example that the numeral 5 is the same as the word five or the five diamonds on a playing card (number sense), the concept of biggest/largest v. smallest, the mechanics of doing maths or applying their maths knowledge to solving a problem.
There are likely to be difficulties dealing with numbers at very elementary levels; this includes learning number facts and procedures, telling the time, time keeping, understanding quantity, prices and money.
Dyscalculia is a continuum and individuals may have slight to very severe difficulties.
While difficulties with numeracy and maths are also common with Dyslexia it is not correct to say that Dyscalculia is Maths Dyslexia. They are different, although may co-occur as they can with other Learning Differences.
This video provides a helpful discussion on Dyscalculia.