Dropbox storage allows you to use your personal Dropbox account to automatically backup videos recorded by your Camera.
Dropbox video storage does not replace the storage on the micro-SD card. All videos remain on the SD card as always and those that you have chosen to back up in the Dropbox settings will be automatically uploaded to Dropbox.
A Dropbox icon on the video player of the Home + Security app indicates that the video has been sent to Dropbox. If you tap on it the video will open in Dropbox, even if your camera is offline.
To learn how to link your Dropbox account to your Camera see this article.