HTTP is a request and response protocol used to enable communications between clients and servers.
Basically, a client send a request to a server on a particular port. The server listening to that port is waiting for the client's request. And when it receives the request from the client, the server sends back the corresponding response.
To secure this communication, a SSL protocol can be used on top of HTTP to provide security. This will allow the data to be encrypted and safely transfered through a secure HTTP protocol: HTTPS.