service key
A service key is a long secret string your program sends with each call, so that the service knows the call is yours. Most software calls it an API key.
In more detail
A person signs in with a name and a password. A program sends its key instead, with no name beside it.
One string does the work of the two. It is long enough that nobody guesses it, it belongs to one account, and it can be replaced without touching the account it belongs to.
What it does on a real document
Every call to the service sends the key with it, and the service counts each PDF document it creates for that key against your plan. It is also how the service tells your fonts from another account's.
Your account page shows the key in clear whenever you are signed in. If somebody else has seen it, replace it there: the old key stops working and a new one takes its place in the same move, and nothing else changes.
Where it comes from, and why
Services that programs call, rather than people, settled on keys of this kind. A password belongs to a person and unlocks everything; a service key belongs to one program and can be taken away on its own.
Of the ways of proving who is calling, this one asks the least of the calling program. That is also its weakness: anyone holding the string can act as you, so it belongs in the settings of your server and never in the code of a page.