When you develop Android Apps, you will require a unique App icon for your Apps. The requirement of the App icon requires a different aspect of the image that fits different size of Android screen. The old method may require some photo processing skills such as using Photoshop or Illustrator.
To help out developers in this situation, Google provides a Web app called Android Asset Studio that allows you to just upload an image and adjust the size to generate correct size for you to import into your App and save time on messing around the App icon stuff.
Here is the procedure.