The export format is a text file (.txt). You can open and read it with a notepad or text editor app on your computer or smartphone. Photos attached to your diary entries are also exported as image files.
Once you subscribe to the Premium Plan, "Data Export" in the settings screen becomes available. Choose "All Time" or "Selected Period" (with a "Start Date" and "End Date") under "Data Period", and each diary entry is exported as a text file and bundled into a zip file.
You can send the zip file to your computer by attaching it to an email or saving it to Google Drive or a similar service.