The "Back up" action saves your local preferences into your backup file (.ssbackup). It overwrites the data contained in the backup if required without looking at their value and their backup date. If you use the same backup file in several machines, you may lose your latest backup data.
At the opposite of the "Back up" action, the "Install" action extracts the saved data contained in your backup file (.ssbackup), and it copies them to your local machine without looking at their value and their last modification date. If you use the same backup file in several devices, you may lose your latest local preferences.
The "Synchronize" action is a combination of the "Back up" action and the "Install" action. The action will compare the data located in the backup file and the data located locally on your machine. It will then decided to back up the settings files or install the saved settings to keep only the most recent. If the data are already synced, the action will do nothing on the selected item.