Regular ("classical") cryptography methods use data blocks called "secret keys". Information encrypted using some "secret key" can be decrypted by anyone who knows the encryption method and posses the same "secret key". This type of cryptography is called symmetric cryptography.
An alternative cryptography method is based on key pairs - a "private key" and a "public key". These keys must be generated together using special algorithms. Any information encrypted with the "private key" can be decrypted by anyone who knows the matching "public key", and any information encrypted with the "public key" can be decrypted using the matching "private key". The Public Key Infrastructure (PKI) is the technology based on this asymmetric cryptography.