You can find detailed information here:
Crash course SSL certificate
First of all: SSL stands for "Secure Sockets Layer" and encrypts the communication of data that is transported from your computer to a server. One often sees SSL / TLS, where the abbreviation TLS stands for "Transport Layer Security". TLS is nothing more than the standardized further development of the “Secure Sockets Layer” protocol from Netscape.
The data to be transmitted between the web browser (or email program) and the server is encrypted, and the details can be found in a corresponding SSL certificate. In other words: The SSL certificate is like a contract between the web browser (email program) and the web server in which the conditions are defined. The main tasks of TLS are:

  • The authentication of the communication partner. Asymmetrical encryption methods are used here.

  • The confidential end-to-end data transmission with the help of symmetric encryption. This is achieved with the use of a shared session key.

  • Ensuring the integrity of the transported data.

  • A digital SSL certificate is a digital data record that confirms certain properties of people or objects and whose authenticity and integrity can be checked using cryptographic processes. The digital certificate contains all the information required for this exam.

How does a TLS / SSL certificate work?
Your client (mail program or browser) establishes a connection to a server and usually the server first authenticates itself to the client with a certificate. Then either the client sends the server a secret random number - encrypted with the server's public key - or the server and your client calculate a shared secret using the Diffie-Hellman key exchange method. A key is then calculated from this secret, which is then used to encode the communication with symmetrical encryption.
An SSL certificate is nothing more than an agreement between your client and a server in order to be able to encrypt your data to be transmitted to the server.
By the way, you can see whether SSL / TLS is activated by a small lock icon that appears in different places in different browsers. Most of the time, the SSL / TLS lock is displayed in the address bar or at the bottom of the browser.