Hyper Text Transfer Protocol. What an important concept, it led to the World Web Web, and the rest is history. Let's take a closer look at something we so often ignore in our daily lives on the Internet.
Here's the structure of URLs (Universal Resource Locators) on the modern Internet:
<protocol name> : / / <domain name> / <file path name>
The protocol name that is most commonly used is HTTP and HTTPS (a secure form of the same), it is so common that we do not generally think of it at all.
The domain names are also common knowledge, such as "google.com" or "facebook.com", as well as the full name of the file or resource including the full path such as /a/b/c/file.html.
Why is all this important? Because HTTP is a protocol, an open interoperable standard that allows all networks on the inter-network (Internet) to talk to each other.
This is what is meant by protocol, a standardized way to do something that is codified into processes and often software that just makes it easy for everyone to rely upon it, and for various parties to leverage interoperability.
The Internet is already built using open standards (TCP/IP for instance: Transfer Control Protocol / Internet Protocol), which are the underlying protocols that deal with physical networking etc, all the way up to session management and the Application Layer, where developers can create functionality that users can access via the web.
This Application Layer (is the seventh layer in the OSI model for the network stack) and is the genesis for the "app economy" which exploded in the late 2000s and led to the Web 2.0 phase of the Internet.