Communications on the internet can be of two types, encrypted or unencrypted. The internet connections can be intercepted by anyone who can be interposed in the connection, or also simply be able to see its flow (for example in the case of computers on the same network). Being a unencrypted communication, those who manage to intercept can see its contents. Think when you log in to a web page.
A malicious user may be able to read the login data, with foreseeable consequences, consider, for example, login data to access a bank account.
Because of the risk to which internet connections are subjected a encryption technology has been developed, called SSL (Secure Sockets Layer) and evolved into TLS (Transport Layer Security) that applied to connections Web / HTTP is called HTTPS. Continue reading