what is this used in oneway encryption that produces an output of a fixed length whatever input is

The term being referred to in the question is a one-way hash function, also known as a message digest, fingerprint, or compression function. This is a mathematical function that takes a variable-length input and produces a fixed-length output, known as a hash value or message digest. The output is unique to the input, meaning that even a small change in the input will produce a completely different output. One-way hash functions are commonly used in cryptography to ensure the integrity of data and to securely store passwords. When a user creates a password, the password is hashed and stored in a database. When the user logs in, the password they enter is hashed and compared to the stored hash value. If the two values match, the user is granted access.