Normally, if DropDMG does not have the privileges that it needs in order to create an image or archive, it will prompt you to authenticate by entering an administrator password. This authentication lasts for a few minutes, or until you quit DropDMG (whichever happens first).
If you choose Always Run With Root Access from the DropDMG menu, DropDMG will retain administrator access—even between launches—until you choose this command again (to uncheck it). This saves you from the trouble of having to keep entering your password, and it makes it easier to run DropDMG unattended (for example, as part of an automated, scheduled backup). However, this is a potential security risk. Anyone with access to your copy of DropDMG will be able to access and modify any file on your Mac.
In order to take advantage of Always Run With Root Access, the DropDMG application should be located on your Mac’s startup volume.