This means you have to use most of the major ones if you really want to find something. The someone who enters the information about the website or other internet resource can be you, or someone else, who owns a website or produced an internet resource , or it can be the people who maintain the search engine, or a combination of both. Yahoo is an example of a directory. Directory search engines don't look for anything on the internet to obtain the information they contain. They get the information from people who enter it into the search engines database.
Some directories collect more or less types of information than other directories.
Each directory has its own methods of categorizing the information. This is why there are so many directories. You may be maintaining your own directory in the form of a list of bookmarks or in a hotlist. If you aren't, you probably will. You don't need to understand what these directories collect or how they organize it.