Ought implies can is an ethical formula ascribed to Immanuel Kant that claims an agent, if morally obliged to perform a certain action, must logically be able to perform it:
For if the moral law commands that we ought to be better human beings now, it inescapably follows that we must be capable of being better human beings.
The action to which the "ought" applies must indeed be possible under natural conditions.
Kant believed this principle was a categorical freedom, bound only by the free will, as opposed to the Humean hypothetical freedom ("Free to do otherwise if I had so chosen"). There are several ways of deriving the formula, for example, the argument that it is wrong to blame people for things that they cannot control (essentially phrasing the formula as the contrapositive "'cannot' implies 'has no duty to'").