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 are deprecated, and all major browsers aim to remove them in Q1 2020. They are already disabled in Firefox Nightly 71 (see bug 1579270). See also TLS 1.0 and 1.1 are now deprecated, disabled in Nightly for more details.