To go into further detail When you level a class getting to certain levels unlock some of that classes skills to be used on other classes. Every 5 levels of your currently equipped class you gain access to 1 additional ability from cross classing (i think it slows down as you get higher but at least 4 @ lv 20). You can then select abilities from any of your leveled classes to use on your equipped class. Not ALL abilities from the other class are available but most of the really important ones are.