First some vocabulary:

• Each of the pieces of content in the right and left columns (ie newsletter signup, navigation area, events, etc) are referred to as portlets in the system.
• The ZMI (Zope Management Interface) is an area that controls several of the settings of the Plone site. When you are in the ZMI, you will notice the content folders are there, as well as several other pages that control settings and behaviors for the site.

In the left and right columns of a page, Plone displays different boxes with different content - navigation, login, news, events etc.

But you normally don't want to display all boxes on all pages. So you have two variables which control box (aka. portlet) displays: left_slots and right_slots.  By specifying a certain portlet in these slots, you will make it appear. By not specifying it, it does not appear. This can also be done for individual pages and sections.

• First, enter the ZMI ( >Plone Setup > ZMI).
• Then, go to the root of the Plone site (in the ZMI). This will be the default page - you can verify this by looking at the breadcrumbs indication on that page, which should read: "Plone site at /".
• Click on "properties" (tab across the top) and add the portlet slot (either to the left slot (ie column) or right slot (column).  The portlets have names such as "here/myPortlet/macros/portlet", where myPortlet is the name given to the portlet (or in the case of static portlets, the page template). After adding/removing the line for the portlet you want to change the display of, press Save.
• From the Plone interface (note that you can change from the ZMI to the Plone by clicking the "View" tab), verify the changes for the portlet in question.