Exporting a backup of your chart is a good idea since the data is not automatically saved in the cloud. That means that if your tablet breaks or gets stolen you don't have to start all over again.
You can also use this feature to move progress between different devices by exporting from one device and importing on another.
To export, navigate to the chart list view and tap the three small dots by the chart name of the chart you wish to export. In the popup menu choose the "Export progress" option.
The app will process for a short while and then popup a dialog where you can choose where to save your exported file. I would recommend somewhere not on the device like email or a cloud drive.
The exported file is a PDF file and you can import it again using the same way as any other PDF file.