Transport Layer Security (TLS), formerly known as Secure Sockets Layer (SSL), is a protocol used by applications to communicate securely across a network, preventing tampering with and eavesdropping on email, web browsing, messaging, and other protocols. Both SSL and TLS are client / server protocols that ensure communication privacy by using cryptographic protocols to provide security over a network. When a server and client communicate using TLS, it ensures that no third party can eavesdrop or tamper with any message.
All modern browsers support the TLS protocol, requiring the server to provide a valid digital certificate confirming its identity in order to establish a secure connection. It is possible for both the client and server to mutually authenticate each other, if both parties provide their own individual digital certificates.
Note: TLS 1.0 and 1.1 support will be removed from all major browsers in early 2020; you'll need to make sure your web server supports TLS 1.2 or 1.3 going forward. From version 74 onwards, Firefox will return a Secure Connection Failed error when connecting to servers using the older TLS versions (bug 1606734).