An SSL Certificate is used to establish a secure, encrypted connection between your domain and a user’s browser, preventing unauthorised parties from accessing or interfering with your data.
Instead of starting with HTTP (HyperText Transfer Protocol), your website address will start with HTTPS (HyperText Transfer Protocol Secure). Using HTTPS, the computers agree on a "code" between them, and then they scramble the messages using that "code" so that no one in between can read them. This keeps your information safer from hackers or other unauthorized users.
This is particularly important when entering personal information from people completing online forms, entering passwords or providing credit card details in online forms. Sites secured by an SSL Certificate show a green padlock or “Secure” message next to the URL in Google Chrome (and other browsers), which reassures users that the site is fully secured and that they are safe to access the site or to enter their personal details.
In simple terms, if your domain has an SSL Certificate and starts with HTTPS, it shows that it is safe and secure and builds customer confidence. If it doesn't, you'll not only be risking GDPR compliance, you'll get fewer people visiting your website as your site will be ranked lower in Google, and visitors/customers will be put off accessing a website that displays a warning message. You'll also probably start to get messages from people seeking reassurance that it is safe to access your website.