As long as you can add and subtract the ``things'' in your matrices, you can add and subtract the matrices themselves. The addition and subtraction occurs in the obvious way--element by element. Here are a couple of examples:
To find what goes in row 
and column 
of the sum or difference,
just add or subtract the entries in row and column of the
matrices being added or subtracted.
In order to make sense, both of the matrices in the sum or difference
must have the same number of rows and columns. It makes no sense, for
example, to add a 
matrix to a 
matrix.