Hashcash is a proof-of-work system designed to combat email spam and denial-of-service attacks. It requires the sender of an email or network request to perform a certain amount of computational work, generating a hash value that meets specific criteria. This proof-of-work requirement makes it costly and time-consuming for spammers or attackers to flood a network with requests. Hashcash provides a mechanism to deter abuse and ensure that computational resources are dedicated to legitimate activities. It served as an inspiration for the proof-of-work consensus algorithm used in Bitcoin and other cryptocurrencies.