Technically it means if you're trying to write an algorithm to play Magic the same algorithm could translated and applied to solving the halting problem (i.e. a reduction of Halt to Magic exists). So your task is that difficult. Thus the theory says it is a logical contradiction for any algorithm to exist that can solve Magic. In practice, this can be different because we are routinely successful in special cases for example we still have anti virus programs in practice, just no one perfect anti virus program can exist for the same reason.