What is mCn?
No of ways of selecting n elements out of m.
Now look at the expression.
Func( m-1, n). It says...I didn't select that element and now m looking for n elements out of m-1.
Function(m-1,n-1). It says...I selected that element and now I am looking for n-1 more elements out of remaining m-1.
An element can be selected or rejected. Both cases covered.