What does the RT in Windows RT mean?

The "RT" in Windows RT stands for RunTime. It is because Windows RT is designed to run on lower power ARM processor tablets. A smaller runtime version of Windows 8 will be used, hence the name. Windows RT won't have the power of a full Windows 8 capable Intel x86 processor computer. But the Windows RT models will be cheaper and capable of doing the same as iPads and Android tablets today.

Here are a couple of interesting links about Windows RT and Windows 8: 

Windows Blog announcement of Windows 8 editions

Wikipedia

NOTE: I wrote this in July 2012. Windows RT, 8 and Surface are due soon, August or September 2012.