Short for release to manufacturing, RTM is the version of software released to hardware manufacturers for bundling. The RTM process is beneficial in that it helps developers work out any bugs before general release via manufacturer refinement of the software. For example, when Microsoft releases an operating system, they RTM the disc, and all future copies of the software are made from that disc. Apple refers to this copy as a GM (gold master).
With Internet software or services, RTW (release to web) describes a product ready to be published on the Internet.