Some problems are undecidable: no computer can solve them, no matter how powerful.
Think of an impossible puzzle: you want to know if a program will ever get stuck, but you can't find out for sure. Turing showed there are questions computers can never answer, even if they are very fast. It's like trying to find a treasure that doesn't exist.
Why it matters This limit of computation helps us understand why some problems, like perfect security, are impossible.