The Internet is just a pile of information. Some of that information is text, some of it video, some of it audio and so on. Some of the information is easily viewable by everybody and some of it is hidden away — either behind passwords or technically embedded in the Web software itself.
To make stuff for the Web, it helps to organize it a bit before you get into the expensive task of coding the software. If you organize well then you’re more likely to have what you need when the coding is done.