When someone makes a claim and puts it on someone else to prove their claim false.
John says that Katie is guilty of theft since Katie didn't respond after John accused her of doing so.
A simple accusation does not prove someone's guilt.
John claims that aliens are secretly taking over the world. Since you cannot disprove it then the claim must be true.
Again, the truth of an argument should never hinge on whether another person is able to disprove it.