The HyperText Transfer Protocol (HTTP) is the underlying network protocol that enables transfer of hypermedia documents on the Web, typically between a browser and a server so that humans can read them. The current version of the HTTP specification is called HTTP/2.
As part of a URI, the "http://" is called "schema" and usually stands at the beginning of an address, for instance in "https://developer.mozilla.org" to indicate to the browser to request the document using the HTTP protocol. The https in this case refers to the secure version of the HTTP protocol, SSL (also called TLS).
HTTP is textual (all communication is done in plain text) and stateless (no communication is aware of previous communications). This property makes it ideal for humans to read documents (web sites) on the world wide web. However, HTTP can also be used as a basis for REST web services from server to server or AJAX requests within web sites to make them more dynamic.