An SSL/TLS X.509 certificate is a digital file that's usable for Secure Sockets Layer (SSL) or Transport Layer Security (TLS). The certificate fulfills two functions. First, the certificate can assist with authenticating and verifying the identity of a host or site. Second, it enables the encryption of information exchanged via a website.
An SSL/TLS certificate is one of the most popular types of X.509 certificates, or a type of public key certificate which uses the X.509 standard. X.509 certificates contain a public key and the identity of a hostname, organization, or individual. Some of these certificates are self-signed. When a certificate authority (CA) signs them or another entity validates them, the owner of that certificate can leverage the public key to establish secure connections with another party or validate documents someone digitally signed using the corresponding private key.
SSL/TLS certificates are X.509 certificates with Extended Key Usage: Server Authentication (22.214.171.124.126.96.36.199.1). The "Extended Key Usage" extension lists the "roles" for the entity that uses the certificate. In other words, an entity must use SSL/TLS certificates only for server authentication and nothing else. Otherwise, that entity risks violating the issuing CA's policies.
There are also other common types of X.509 certificates, like Client Authentication (188.8.131.52.184.108.40.206.2) and Code Signing (220.127.116.11.18.104.22.168.3). These files form the basis of encryption and authentication schemes.
As SSL/TLS certificates enable encryption, they are integral to Hyper Text Transfer Protocol Secure(HTTPS), a protocol which encrypts all communication exchanged between a website and your browser.
You can check that a website you're visiting is using HTTPS by looking for "HTTPS" in the address bar. There should also be a padlock symbol next to the website's address. If you click on that symbol, your web browser should display the name of the organization that owns the SSL/TLS certificate. That symbol turns green when your web browser detects an Extended Validation (EV) SSL certificate.
If the certificate has expired, the web browser will display an error message or warning. These alerts could lead a visitor to navigate away from a website. To prevent this from happening, organizations that own websites and use HTTPS need to manage their certificates and make sure the ones they want to keep don't expire. Are all your certificates up to-date?