An invoice is a formal claim for payment: the supplier stating what was supplied, what is owed and by when. Everything after it is the buyer establishing whether the claim is correct and authorising it. Seeing it as a claim to be verified, rather than a bill to be settled, explains most of what payables teams do and why suppliers experience the delays they do.
What the supplier puts on it
Their details and yours, a unique invoice number, the date, a description of what was supplied with quantities and prices, tax, the total, and payment terms. Many carry a purchase order reference, and that single field is the difference between an invoice that can be checked automatically and one that has to be investigated by a person.
What the buyer does with it
Checks it is genuine and addressed to the right entity, checks the number has not been seen before, compares it with what was ordered and what arrived, routes it to whoever has authority for that spend, records the approval, codes it and schedules payment on terms. Each step is short; the routing step is where the days go.
Why payment takes longer than the work
Almost never because anybody is deliberately holding money. Usually because the invoice could not be matched, went to an individual instead of a monitored address, or waited on an approver who was unavailable. Suppliers can shorten two of those three simply by quoting the order reference and billing the right address.
Questions people ask about how does an invoice work
When does the payment clock start?
Usually on receipt of a correct invoice by the buyer, which is why the arrival date matters and why disputes reopen the conversation about terms.
What makes an invoice hard to process?
No purchase order reference, the wrong legal entity, or amounts that do not correspond to anything ordered or delivered.
Why do buyers insist on a reference?
Because it connects the invoice to the order and the receipt, which is what lets it be checked without a person investigating.