What is a witness string in a zero knowledge proof?

I had earlier asked the question What is a witness string? I unable to understand the concept

From the answers I got, it seems that a witness is more or less the answer. For e.g. in the padlock example given in the accepted answer, the witness is the padlock combination which can be easily verified to be true or false.

I assume it (witness == answer) is not the same in Zero Knowledge proof systems because then it wouldn't be a zero knowledge system. Can someone explain witnesses in the context of Zero Knowledge Proof systems?

In the padlock example, if we had to provide a zero knowledge proof, what would be a witness string? Alternately is there some other simple example of ZK proof, where we can understand what a witness string is?

A witness $$w$$ is essentially some extra helping information that you can use to verify something.
In the case of zero-knowledge proofs, $$w$$ will help you prove that the other party solved the problem, but it may not necessarily be the solution (since we need the zero knowledge property).