RTM is short for release to manufacturer, and it's ready two weeks prior to final release on July 29th. The build that has been submitted as final has version number 10240.

The RTM version is intended for manufacturers who install Windows on their devices like laptops and PCs.  The RTM status is the last phase for the public release of the OS. Microsoft announced Windows 10 last year and has been releasing public test version that have been well received.

Obviously for us PC gamers DirectX 12 is going to be a hip and hot feature. Microsoft has promised that all users of Windows 7 and 8.0 can get a free upgrade to Windows 10, you will need to upgrade this year though as that is the one condition set by MS. Due to the free upgrade the adoption rate of Windows 10 will likely go through the roof. 

A fun fact, Microsoft releasing the RTM precisely today is not a coincidence, today it has been exactly 20 years ago that Microsoft send Windows 95 RTM to manufacturers.