Public Key Infrastructure (PKI) is a technology for authenticating devices and users in the digital world. The basic idea behind PKI is to have one or more trusted parties digitally sign documents certifying that a particular cryptographic key belongs to a particular user or device.
Nowadays most basic authentication involves a private key (also known as a secret key), which only one person should be able to use to decrypt those messages and a public key, which anyone can use to encrypt a message. PKI govern encryption keys through the issuance of digital certificates and their management therefore verifying the owner of the private key.
SSL certificates on websites are one of the most common examples of PKI.