The export option is available from the 'History' section. Exporting your transactions history is very useful if you want to perform analyses about your operations.
In 'History', you can filter transactions depending on their payment method, their date, their issuer, etc.
When exporting, you have the choice between downloading all your transactions or only those you've filtered. Also choose to download attachments or not. Click on 'Export' and set your preferences to get your csv file.
Please note that there's no time limit for exporting your transactions, you can dowload all your operations since the creation of your account if you want to.