This commonly used initialism stands for Hypertext Transfer Protocol. It is used to describe the underlying mechanics of how client browsers and web browsers transfer information, including webpages and their associated files.
The key feature of HTTP, and its associated markup language HTML (Hypertext Markup Language), is to allow for hypertextual links between different webpages. These links can be between webpages on the same website, or between different websites entirely. The internet was designed with this feature in mind in order to create the sensation of surfing or jumping from one link to another to search the interment for content.