First, what is an SSL certificate and what is the HTTPS protocol?
The best comparison, in my opinion, is the passport. A passport is an identity document issued by the Government of Canada (a trusted third party) that attests to the identity of its holder (name, date of birth, photo, etc.). When presented at a country’s border, the passport lets the holder enter the country. In the same way, the SSL certificate is like an identity card. It is signed by a trusted third party that attests to the identity of the website (a physical and digital entity). When installed on a web server, it activates the padlock and the HTTPS protocol, ensuring a secure connection (data encryption) between the browser and the website (server). 
In the past, SSL certificates were used by transactional sites to secure the data provided by the user (name, address, phone number, credit card number, login, password) during a purchase. Today, the use of certificates is strongly recommended for all websites you can log in to with a username and a password. This applies just as much to social media sites as to your company website built with WordPress or SPIP. Google has in fact announced that, starting in January 2017, Chrome users will see information in the address bar telling them whether or not the site is secure when they are on a login page (username and password). The goal of this change is to raise awareness of the importance of using SSL certificates for any form. 
Why do the sites I log in to need an SSL certificate? What does it protect?
The main reason is to ensure that sensitive information (password, credit card, name, address, etc.) sent over the internet is encrypted so that only the intended recipient can read it. This is important because the information you send over the internet is passed from one computer (server) to another to reach the destination server. Any computer between you and the server can display your credit card numbers, usernames and passwords, as well as other sensitive information, if it is not encrypted with an SSL certificate. When an SSL certificate is used, the information becomes unreadable to everyone except the server you are sending the information to. This protects it from hackers and identity thieves.
How do I add an SSL certificate to my website?
Contact the company that hosts your website and request one. All hosting providers offer SSL certificates, at very competitive prices (between $50 and $150/year). It will probably be necessary to make a few adjustments to your website after installing the certificate to make sure all links include the HTTPS protocol. Do not hesitate to contact us for more information at info@zaa.cc!