Hash
In short: A hash is the fixed-length output generated by a hash function, an algorithm used to secure and verify data.
A hash is the fixed-length output generated by a hash function, an algorithm used to secure and verify data. Whatever the size of the input, the hash is a short string that acts like a digital fingerprint of that data.
Why it matters
Good cryptographic hash functions have a few key properties. The same input always gives the same hash. Changing even one character gives a completely different hash. It is practically impossible to work out the input from the hash, or to find two different inputs with the same hash. Hashing is often described as encrypting data, but it is one-way: there is no key, and the data cannot be recovered from its hash. Blockchains rely on hashes everywhere. Each block header contains the hash of the previous block, which links blocks together and makes tampering obvious. Transactions are identified by their hashes and summarized in a Merkle tree, and proof-of-work mining is a search for a block hash that meets a target. Bitcoin uses SHA-256, and Ethereum uses Keccak-256.
Example
The SHA-256 hashes of "hello" and "Hello" look completely unrelated, even though only one letter differs, and each is 64 hexadecimal characters long. When an exchange shows a transaction ID (TxID) for your deposit or withdrawal, that ID is a hash you can paste into a block explorer to find the transaction.