A Drawable resource is a general concept for a drawing-able picture. A graphics file (bitmap) is the most basic instance, which is represented in Android by the BitmapDrawable class.
In one of the res/drawable folders, each Drawable is saved as a separate file. Bitmaps for various resolutions are often stored in the res/drawable subfolders -mdpi, -hdpi, -xhdpi, -xxhdpi. By default, the ADT project creation wizard creates these directories. If these bitmaps are provided in separate folders, the Android system automatically selects the relevant one based on the device setup.
The Android system scales the closest fit up or down if you don't offer bitmaps for all supported resolutions. This is usually undesirable since the bitmap may get blurry.