You've probably been asked that question by now. And if you're like me, you cannot put together a satisfying answer. A demonstration does the trick, but a description that conveys what HyperCard IS, seems out of reach. Computers are called machines.
Most machines have a specific function and purpose.
But the kind of machine computers ARE changes with each program they run. At the highest level of operation, the computer is many different machines, and what it can become can never be exhausted. It might be better to call computers meta-machines. Computers present packages of possibilities for carrying and giving shape to information, which is always at a higher level of reality than matter. HyperCard, like a computer, is a blank slate too, modifying the Macintosh with software, presenting the stack author and reader with a rich field of possibilities.