What does windows RTM mean?

The term, RTM stands for "release to manufacturing," it explains the time at which a product team, in this case the Windows team at Microsoft, signs off on the final release code for the product in question, in this case Windows. And "signs off on,"  literally means "signs off on": There is an often-oversized board—kind of like those novelty checks you see lottery winners receive—on which all of the employees in charge of key components of the product sign their name. The idea, is that by providing their personal guarantee that their part of the product is both complete and fully functional that the overall project is of course of high quality as well. They're taking personal responsibility for it.

Here are the Software Release Form boards for Windows 2000 Professional and Windows 2000 Server  :

In the old days, the RTM process also involved the creation of what was called "the golden master," an optical disc containing that final RTM code. So named because the disc was literally gold (as CD-R and then DVD-R discs were back in the day), the golden master(s) were created—thus the "manufacturing" part of RTM—and then physically delivered to Microsoft's PC maker partners, often with great pomp and circumstance. In fact, former Windows lead Jim Allchin once showed  the Windows Vista kit that the firm gave to PC makers to celebrate that release. It looked like the type of explosive case a spy chains to his arm in an action movie.

source: http://winsupersite.com/windows-8/rtm-whats-name