The name of the plugin is self-explanatory. It simply removes empty vector layers (i.e. layers with no features) from the map.
If you are a user you might find it useful when you load at once a large number of layers to the map (for instance, by using the "Load them all" plugin) and afterwards you want to get rid of those layers that don't have anything to be displayed, because they are empty. So you just have to click on the plugin's button and you are done.
If you want to start learning how to develop QGIS plugins, this could be a good resource for you because it is simple and because I spent some time for details such as translations (English and Spanish are supported), appearance (the icon is changed according to the theme you choose for QGIS) and QGIS version changes handling, namely, to deal with a method (getLayerID) that is deprecated since v.1.7.
You can access to the plugin via the QGIS plugin installer or via the QGIS plugin repository: http://plugins.qgis.org/plugins/RemoveEmptyLayers/