PrestaShop lets customers save their shopping cart between visit sessions, so they can finalize their order at a later date.
This feature is very practical, but consumes space in your store's database.
The simplest solution would be to be able to delete shopping baskets directly in your store's backoffice, but this is not a standard feature in PrestaShop.
However, anyone can activate this feature.
Simply open the file /your-admin/tabs/AdminCarts.php and modify line 25 by replacing $this->delete = false; with $this->delete = true; and then you can delete baskets individually by clicking on the red cross at the end of each basket's line, or by selecting them and deleting them en masse to go faster.
Under no circumstances should you delete baskets corresponding to orders, as this will cause errors in certain functions, such as PDF invoices.