SSL certificate in practice
An SSL certificate is a file installed on the server that lets the website use an encrypted connection. When it is active, the address starts with https instead of http and the browser shows the padlock. Today the correct protocol is called TLS, but people still say SSL: it means the same thing.
Without a certificate, browsers flag the site as «not secure» right next to the address. For a visitor that is reason enough to close the page, especially if they are asked for their name, phone number or payment details. Google also treats https as a basic requirement, so a site without it starts at a disadvantage in SEO too.
A practical example: a dental practice has a form for booking appointments. With https, the patient's name, phone number and reason for the visit travel encrypted, and nobody on the same public Wi-Fi network can read them. For an online shop the certificate is simply essential: payment systems require a secure connection.
Many web hosting plans include free certificates that renew themselves, and for most websites they are enough. What to check: that renewal really is automatic, because an expired certificate stops visitors with a warning page; that every version of the address, with and without www, leads to the https version; that images and scripts are not still loaded over http, otherwise the padlock disappears.
