HTTPS is when HTTP (hypertext transfer protocol); a communication protocol, is encrypted using TLS (transport layer security).
This means that if anyone is eavesdropping on the communication between a server and a client, they will eavesdrop on encrypted data that is hard to decipher and is therefore secure.
TLS is the more recent term which replaces the term SSL (secure socket layer) but we can refer to them as SSL/TLS.