A hash is a result of a cryptographic function that converts any input, such as a transaction, file, or block of data, into a unique string of letters and numbers of fixed length. No matter how large or small the original data is, its hash output will always be the same size.

Hashes are fundamental to blockchain security. Each block contains the hash of the previous one, creating a tamper-evident chain: if any information changes, the hash changes completely, signaling that the data was altered. This property allows blockchains to verify information, secure transactions, and prevent fraud without revealing the original data.

In simple terms, hashing ensures that blockchain data is verifiable, consistent, and secure, forming the backbone of trust in decentralized systems.