In telegraphic terms, it is information that allows a cryptographic activity to be performed. This can be, for example, the encryption of messages, their decryption, electronic signatures or their verification.
Symmetric cryptography uses a single key, the same for both parties, so only the participants in a particular information exchange should know it.
Asymmetric cryptography uses pairs of keys – public and private. The former is generated by mathematical methods from the latter, and reversing such a process should be as mathematically difficult as possible. The most commonly used way to create such keys is currently the RSA algorithm.