A certificate or digital certificate is a unique, digitally signed document which authoritatively identifies the identity of an individual or organization. Using public key cryptography, its authenticity can be verified to ensure that the software or website you are using is legitimate. On the Internet, a certificate is signed by a trusted CA (certificate authority), and verified with the authority's public key. The decrypted certificate contains a verified public key of the certificate holder (website operator), with which encrypted HTTPS communications can be established.
An operating system or web browser may alert the user when loading software or a website whose digital certificate is not verified by a trusted CA.