Goto any particular board page and click the options in the board header for viewing board in the list view .
You can export all card details by clicking Export button from the Export CSV dropdown and you can select which card details you want to export in the form of CSV file.
You can also export particular cards details by using filter cards option in the board header settings menu .
Afer filtering cards, click Export button in the Export CSV dropdown menu for exporting the filtered card with selected d fields in the Export CSV dropdown menu
Now when you click export button, the card details will be exported as CSV file and the CSV file content will be represented in the given below image format.