Dropsync is a two-way sync client. Once a folder on your phone's or tablet's SD card and another one in your Dropbox are linked together any file additions, changes, deletions on one side will be propagated to the other side. This is similar to how Dropbox works on Mac or PC. Sync happens automatically in the background depending on network and power source (AC/battery) conditions. The conditions are of course configurable.
You need to pick at least one folder on SD Card and link it with a folder in Dropbox. We'll do it in the next few screens.
Tip: one of the folders on SD card worth being synced with Dropbox is DCIM. This is where all the photos you captured by the phone camera are stored. If you have multiple Android devices, say a phone and a tablet, try to configure both of them to sync photos with the same folder in your Dropbox account. That way you would have all your photos on any device you own regardless which one was used to take the photos.