The HTTP protocol is the Hypertext Transfer Protocol.
A protocol is used to regulate/constrain a certain class of things.
Wondering what kind of things the HTTP protocol is used to regulate/constrain? Need to know about communication first?
The most common problem in communication is language barrier. If you want to communicate, you need to stipulate: whether you want to use the same language; whether you want to communicate with each other through a transfer station; these two points can be regarded as an agreement for communication.
So the HTTP protocol is to regulate/restrict how the browser and the server communicate.