Drawable is the virtual resource that can be used as a background, title, or in other parts of the screen. It is compiled into an android.graphics.drawable subclass. A Drawable resource is a general concept for a graphic which can be drawn. The simplest case is a graphical file (bitmap), which would be represented in Android via a BitmapDrawable class.
The Drawable is stored as individual files in one of the res/drawable folders. The ADT project creation wizard creates these folders by default. You would store bitmaps for different resolutions in the -mdpi, -hdpi, -xhdpi, -xxhdpi subfolders of res/drawable If these bitmaps are provided in a different folder, the Android system selects the correct one automatically based on the device configuration.