watermark
A watermark is wording printed across the page itself, usually to say what the document is: a draft, a copy, or a file made on a free account.
What the word means
It is not a layer sitting on top that a program could switch off. It is drawn into the page along with everything else, usually large, pale and at an angle, so that it is read at a glance without stopping the page being read.
Its job is to travel with the document. A page marked draft stays marked when it is printed, photographed or forwarded, which a note in a message does not.
What it does on a real document
Every document made on the free plan carries one saying so. It cannot be taken off the file afterwards; a paid plan simply never puts it there.
The same mechanism prints a draft or a copy mark across your own documents, at the angle and the paleness you choose, above or below what is already on the page.
Where it comes from, and why
The word comes from paper making. A design was worked into the wire mesh a sheet was formed on, so the paper was thinner there and the design showed when the sheet was held up to the light. Paper mills used it for centuries to say who had made a sheet.
It was never only a maker's mark: it was hard to imitate, which made it a way of telling a real sheet from a forged one. Printing wording across a page keeps the second purpose and drops the first — nobody is holding a screen up to the light.