Looking up the google dictionary, repudiation means:
Non-repudiation refers to assurance that someone cannot deny something.
In the example of registered mail or court order, one cannot deny receiving the mail and order when he or she is served with it.
In the electrical world, digital signature servers the purpose of non-repudiation which ensure the document is electronically signed by the person and he/she will not be able to deny the that signature.
In the testing area, non-repudiation is one of the principles tester verify in security testing.