Metadata is information about files. For example, there is no way to know what the file named “Crazyheart.pz2” actually is. Is it pose?… is it a morph?… does it work with Victoria?… does it have something to do with a heart? Your guess is as good as mine! But by using the content_management_service to assign information (metadata) about each file, you can know the answers to these questions. After assigning metadata to “Crazyheart.pz2” you can know its a materials_preset for a cartoon style hamster named Fluffy (just what you were looking for, right?).
It's important because the DAZ Studio 4.x Smart Content pane (and the Carrara 8.5 smart_content pane) only find files that have metadata assigned to them. All of the products in the DAZ 3D store will eventually have metadata. If you have products or files from another source that do not provide metadata, you will not find them in the Smart Content Pane (Tab), and will have to use the Content Library pane to find them. You can add your own metadata by using the Content DB Editor... action. Also, if you have bought products from the DAZ 3D store prior to those products having metadata assigned to them, DAZ will eventually provide an easy way to update the metadata for those products when metadata becomes available.