Part of understanding cryptography is understanding the cryptographic primitives that go into any crypto system.

Explanation

Key

https://en.wikipedia.org/wiki/Key_(cryptography)

In cryptography, a key is a piece of information (a parameter) that determines the functional output of a cryptographic algorithm. For encryption algorithms, a key specifies the transformation of plaintext into ciphertext, and vice versa for decryption algorithms. Keys also specify transformations in other cryptographic algorithms, such as digital signature schemes and message authentication codes.