This is a Latin phrase which means: as much as he deserves. Quantum merit is an equitable doctrine that allows a party to recover for the value of his labour and materials furnished to another even though there was no actual contract so that the other party will not be unjustly enriched.
This doctrine is based on implied promise. Put differently, where there is a breach of a contract, the injured party instead of suing for damages may claim payment for what he has done under the contract.
The right to payment does not arise out of the original contract, but is based on an implied promise by the other party arising from the acceptance of an executed consideration.