An SSL/TLS certificate is a type of digital certificate used to secure the communication between a browser or other applications and a website or web service.
The SSL/TLS certificate confirms the identity of the website and enables encryption so data stays private. The certificate is a small digital file issued by a Certificate Authority (CA). The certificate is digitally signed to prove it is valid, genuine and trusted. To what extend a certificate is trustworthy depends how trustworthy the CA is. Technically you could create your own certificate by yourself but then the certificate is probably not trusted by a browser or other applications as they may not trust you because you are not an official CA. For internal use cases a self issued certificate may still work well to enable encrypted communication between your own servers.
A CA typically checks in an automated way that the requestor of a certificate has technical access to the domain. This prevents that someone could request a certificate for a domain he is not authorized with. The certificate is bound to a domain and includes validity dates. Therefore, the certificate has its own expiry date and needs to be renewed periodically.
A certificate from a trustworthy CA helps to prevent fake websites or phishing attacks, enables encryption, and ensures that data was not changed when travelling between applications and servers.
A very popular and trustworthy CA is Let's Encrypt (What is Let's Encrypt?). Let's Encrypt is a free, automated, and open Certificate Authority (CA) run by the non-profit Internet Security Research Group (ISRG). It provides TLS/SSL certificates to enable HTTPS encryption on websites at no cost, aiming to make the web more secure and privacy-respecting. The process is fully automated, removing complex manual creation and renewal steps.
Anyone with a domain name can obtain a trusted certificate from Let's Encrypt and their certificates are trusted by all major browsers.
