SSL and TLS are the protocols that encrypt traffic between a browser and a server.
THE NAMES
SSL was the original protocol. It was replaced by TLS years ago and every current version is TLS. The name SSL persisted in common usage, so "SSL certificate" means a TLS certificate. Both terms refer to the same thing in practice.
WHAT A CERTIFICATE DOES
Two things. It provides the keys used to encrypt the connection, and it asserts that the server genuinely controls the domain name it claims.
WHAT ENCRYPTION PROTECTS
Anything travelling between visitor and server: passwords, form submissions, payment details, the pages themselves. Without it, anyone positioned on the network between the two can read and alter the traffic.
WHAT IT DOES NOT PROTECT
Data once it arrives on the server. A site with a valid certificate can still be running vulnerable software, storing data carelessly, or serving malware.
WHY IT IS NOW MANDATORY IN PRACTICE
Browsers mark sites without it as not secure, search engines prefer secure sites, and payment providers require it. A site without HTTPS loses visitors before they read anything.
