What is a hash?
A hash function turns any amount of data into a short, fixed-size fingerprint. The same input always gives the same hash, the smallest change gives a completely different one, and you can't work back from a hash to the data. That makes hashes useful for checking that a file arrived intact, spotting duplicates, and storing fingerprints of data without the data itself.
Checking a download
Many downloads publish a SHA-256 checksum next to the file. Choose the downloaded file here, paste the published checksum into "Compare", and you'll see whether they match. The file is read in your browser, a piece at a time, so even very large files work and nothing is uploaded.
Which algorithm?
SHA-256 is the usual choice today, with SHA-512 and the SHA-3 family as strong alternatives. BLAKE3 is a modern, very fast design. MD5 and SHA-1 are broken for security: people can make two different inputs with the same hash. They're still fine for spotting accidental corruption, and still widely published, so they're here for checking checksums.
HMAC
An HMAC is a hash mixed with a secret key, used to check that a message came from someone who knows the key, for example to verify webhook signatures. Enter the key to get HMACs instead of plain hashes. BLAKE3 has its own keyed mode instead, so it's left out when there's a key.
Hashes and passwords
Don't store passwords as plain hashes like these: they're fast to compute, so they're fast to guess. Password storage needs a deliberately slow function such as Argon2, scrypt or bcrypt.