It's not the same for all blockchains. A protocol defines the way they operate.
Basically, a protocol is a simple layer of code that tells something how to work. It's the framework that forms the software base of any given network. Think of a protocol as a collection of rules that allow information to be shared and transmitted by individuals.
The protocols are not crypto-currency specific. In certain places, they live.
Protocols on the Internet make it possible for websites to work. HTTP and HTTPS are the most popular Internet protocols, while TCP/IP and SMTP may also be visible.
The underlying code that allows all Internet applications to run is those protocols. Almost every website you use runs on one of these internet protocols; Facebook, Amazon, Twitter, Google, Netflix, banking websites, news websites.
Cryptocurrencies have protocols, like the internet does. Any cryptocurrency has its own distinct protocol-Bitcoin, Ethereum, XRP.
Cryptocurrency protocols only allow a few applications to run, often only the cryptocurrency application itself. Protocols provide the security of a blockchain and its connectivity.
Historically, to protect the accuracy of our bank account and ensure that our money is not "double spent," we had to trust a bank. This is conventional financial management's centralized model.
But protection is given by the protocol in the decentralized environment of the blockchain, which enables data to be communicated.
As the digital world has evolved, we have allowed our data to be held and viewed by more and more organisations. Across several websites, we have several paying accounts.
Having personal data in so many ways makes it difficult to preserve the accuracy of our data ("What is my email password?") and, in the case of a hack, makes our information vulnerable.
Cryptocurrency protocols allow their data to be handled by users. They allow people to build an account or a wallet on a protocol that can then be used on other websites to pay for services and make financial transactions.
The protection and unique identity of these applications is contained in the protocol at its fundamental level. The protocol, while also providing protection, powers the applications above it.
Why is this cool?
It means you own your unique data and have accessto a range of products and services more powerful and secure.
The major difference between internet protocols and cryptocurrency protocols is that the former normally requires one to a few applications to work, while the latter offers more applications to run.
This is why protocols for cryptocurrencies are called fat: they have very few apps running on them. Internet protocols are thin: they have thousands and thousands of applications that run on them, many millions.
For every crypto-currency, the value is the protocol. It's what the blockchain drives and helps the cryptocurrency to do what it's supposed to do.