"Ought implies can"

Professor Johnson briefly mentioned the quote "ought implies can" in class on Friday. I was confused at first as to what it meant, but after some discussion in class, I had a better understanding. My take on it is that if one has the power to do something, he or she can do it. One example was if we have the power to create world peace, we can make it happen. However, I had one problem with this definition and example. How does one know what he or she has the power to do? There is trial and error, but in some cases, people might believe they have the power to do something but in reality they don't. This could lead to much worse consequences than just a simple failure. If one ought to do something, he or she ought to know their limitations.