An SSL certificate is a small digital “document” that turns the connection between a visitor and your website into a private, sealed conversation that no outsider can read. When a site has one, the browser shows a little padlock and the address starts with “https” instead of “http”.
Picture sending a letter: without SSL it is a postcard that anyone handling it can read; with SSL it is inside a sealed envelope that cannot be opened unnoticed. This matters most when people type passwords, personal details or card numbers — the certificate scrambles that information, so that even if someone intercepts it, they see only a meaningless jumble of characters.
Today it is considered basic for any website: browsers warn visitors away from sites without it, and search engines give a small preference to sites that have it. A certificate is issued for a specific Domain and needs to be renewed from time to time.