How to create monthly bills automatically?

Alright! but I have seen a template named "SALES INVOICE TRACKER". They used a very complicated formulas in it.

It says "=IFERROR(INDEX(InvoiceDetails,SMALL(IF(InvoiceDetails[Invoice '#]=rngInvoice,ROW(InvoiceDetails)-ROW(InvoiceDetails[#Headers])), ROW(1:1)), MATCH($C$13, InvoiceDetails[#Headers], 0)),"")"

If you can explain it to me? I might get what i want.

Thanks

P.S. I'm not familiar with Microsoft Access or you can say that I'm afraid of it :/