The first line for your Item Description is already displayed in the Invoice. If you wish to add more lines just click the "+ Add More" button in blue in left.

If you wish to delete a particular line just click the delete icon in red on right of the same line.

Please note that there is a limit of new rows/lines for each Invoice. The maximum row count is 50. If you wish to add more details than the allowed rows you will have to create a new Invoice.