What it means
Repudiation occurs when a party shows, by words or conduct, that they are unwilling or unable to perform their obligations under the contract, or to perform them only in a way substantially different from what was agreed. Faced with repudiation, the innocent party can usually choose to either accept it and terminate the contract (and claim damages), or affirm the contract and insist on performance. Treating a contract as ended without a genuine repudiation can itself be a breach, so the threshold is high.
How it's used
When the contractor emailed 'I'm walking off this job and won't be back', that was a repudiation entitling the owner to terminate.