Droplets are small, independent, applications that automate common tasks. You save an action and the correct parameters into such a droplet application and every time you drag some files on the droplet the files are automatically processed according to these settings.
This feature allows you to automate frequent tasks such as adjusting file creation dates to match the dates that your photos were actually taken. All you need to do in order to apply the settings is to drag the files onto the droplet icon.