When creating public content, you may of course simply create a new document -- see Help:Adding a Page.

However, if you intend to have many pages on a single topic, you may want to group them under one namespace.  For example, if you intend to have a large number of pages about baseball, you might make a Baseball and Softball page, and then a series of underlying pages such as Baseball:Pitching, etc.

When creating these special groups, they may be public or private.  If private you must create a new Group and administer it.  Your pages would then fall under Group:Baseball:<pages>.  If public, simply place them at the top level with no "Group:" namespace prefix, e.g. Baseball:<pages>.