Imagine asking someone older for some guidance: What can I do in life? Anything you want. Really? Surely, I can't do absolutely anything? If you imagine you can't do much, you won't do much. The only limit / limitation is your imagination.

It means that nothing going to stop you except your imagination, so if you have a wild imagination then nothing will stop you, but if you don't have an imagination then it would be a limit for your strengths and abilities because you don't have enough imagination that helps you to pass through your problems and your challenges.