This dialog is shown as a result of importing or retrieving transactions.
All transactions available for update will be listed. To not update a particular transaction remove the check mark next to that transaction by clicking on the check box with your mouse. Click on it again to turn the check mark back on. You can also use the "Select All" and "Select None" buttons to check/un-check all transactions.
You can sort the list by any column by clicking on the column header. Click it again to toggle between ascending/descending. You can re-order the columns by dragging/dropping a column header to the new desired location.
Choose the "Automatically Record Corresponding Entries in Default Cash Account" to have all the proceeds from selected transactions automatically be added/removed to/from the associated default cash account.