Use this command to select an existing component from the adjacent list box.  A component is just a named group of faces making up a part of the current project.  Select one of the components displayed in the list box by double clicking on its name, and all its member faces will be selected.  The component can then be moved, deleted, replicated, etc -- as an entity.

The list box gives you an overview of all the components in a project.  The Select Component command can also be accessed from the Select Toolbar, and the Background and Face Context Menus.  If a component is already selected when the command is invoked, its name is highlighted in the list box. 

A much easier way to select a component is to double click on one of its member faces in the 3D View.  All its faces will be selected and its name displayed in the Status Bar.