Key is secret information is the secret information that is used by the cryptographic algorithm when creating and checking the digital signature. Usually for digital signature the pair of keys is used:

Private key this key is known only for the owner;

Public key this key is available for all users of cryptographic system.

 

In Digital Signature algorithms the signature is signed on the secret key of a user and is checked on the public key. So anyone may check what user put this signature. Keys are bound with specific certificates.