The underpinning concept of PKI, where one person encrypts a message knowing that the recipient has the secret key to decrypt it, has been in practice for thousands of years. Today, it’s most familiar use is probably online communications. To ensure data safety when communicating with others, the data is encrypted and can only be decrypted by a party with the correct private key.
Guaranteeing the safety of data and authenticating user identity are the biggest security concerns in the ever-growing online universe. Public key infrastructures improve security in both of these situations and are becoming increasingly important for performing many functions.
If an entity is in breach of the policy specifications, the CA will revoke its authenticating certificate, and web servers will flag it as potentially untrustworthy. The actual process is very complex featuring algorithms and ciphers but those are the basic PKI fundamentals explained.