If it walks like duck and quacks like a duck
Duck-Typing is a concept of exhibiting a certain behavior (implementing an interface, following a protocol).
If an object behaves in a certain way it could be considered as a good-enough duck.
This is how groups, societies and organizations work (separation of concerns and qualification).
What it takes to be a…
Substituting of equivalent for an equivalent
In molecular biology the exactly same molecular structure (not a copy, but a part of a larger one) could be "used" as an equivalent.
An implementation of an interface allows an object to be substituted (used) in place of other object.
The canonical example is Iterators. It is a standardized within a language duck-typed interface - a protocol. Everything which implement the interface (protocol) becomes Iterable and could be used everywhere an Iterable could be used - looping constructs, conditionals, high-order procedures, etc.
Another example are ranges, slices, etc.