The term "PAYMENT AGAINST INVOICE" appears on my job contract and no idea what that means exactly.
I am in Germany.
Payment against invoice means they will pay after you send them an invoice (typically after services are rendered), rather than automatically after delivery (PAYMENT AFTER DELIVERY) or ahead of time (PAYMENT IN ADVANCE).
Typically with invoice-based payment you would have a term that said what day they will pay you by. For example, NET 7 would indicate they will issue payment a week after invoice is received, NET 90 would be three months later.