The "M" means there is a value stored in the independent memory. The "D" stands for degrees.
"Math" means the display format is "natural display".
The numerical values for your answers will be correct; unless you pressed the "M+" button as part of your workings.
Don't worry about it - Seems like a pretty pointless function - and a pointless thing to display! I'm doing A2 Maths & Further Maths and had to look it up in the manual, so it's unlikely you'll have deliberately used it incorrectly, unless you pressed "M+" - which, firstly, you'd have no reason to press; and secondly, would be fairly obvious if you had!