Basically anything you can draw/paint on(*): a layer, a layer mask, or a channel. In an image, there is only a single active drawable at any time, so if you can't paint on a layer, it is possibly because you selected a channel, for instance, Select > Save to channel creates a channel from the current selection, and makes it the current drawable.
When Gimp calls a script/plugin, the second argument is often a drawable, and if that makes sense the plugin should be able to work with that.
When Gimp calls a script/plugin, the second argument is often a drawable, and if that makes sense the plugin should be able to work with that.