Excel formule to check the charter invoice

Hi HansVogelaar 
A question due to lack of knowledge... what means the "+1" in de formule? Why need to be de C6+1

=IF(F6="delivered"; INDEX(tblParcels;C6+1;MATCH(E6;tblParcels[#Headers];0))+(INDEX(tblPallets;D6+1;MATCH(E6;tblPallets[#Headers];0))); "")

This formula works. Thanks. I would like only to understand the +1.

(...)
On the second formula - one of the bill column gave me the message #N/A for some rows..
Is the " #N/A message"  due to the fact that my company's data reference are not in the charter list?

My work computer has Excel for Microsoft 365 MSO (version 2307)