status code
A status code is the three-digit number an answer opens with, saying how the call went before anything else is read.
In more detail
The first digit is the whole story. Two hundred and something means it worked. Three hundred and something means look elsewhere. Four hundred and something means the call itself needs changing, and five hundred and something means the machine answering has something to put right.
The rest of the number narrows it down, and the answer usually says in words what the number says in digits. A program reads the number; a person reads the sentence.
What it does on a real document
A request that succeeded answers two hundred, and the body of that answer is the PDF. Anything else answers with a sentence naming what stopped it, which is worth writing into your own log word for word.
Four hundred means the description asked for something the engine could not use, four hundred and two means the request asks for something a licence key covers and the account has none, and four hundred and thirteen means either the body of the call ran past the size the service accepts or the document ran past the pages it allows.
Where it comes from, and why
The numbers were chosen when the first version of the protocol was written, and grouping them by their first digit was the point: a program that has never met a particular number still knows whether to retry, to look elsewhere or to give up.
A few of them have become common speech — the one for a page that does not exist is printed on tee shirts — which says something about how long a good convention lasts.