This archive contains almost everything you need to build the IOMD (RiscPC, A7000, RPCEmu) development ROM image. It includes the build environment and full source code (including all of the latest changes in CVS). The archive should be unpacked using UnTarBZ2, available from the Miscellaneous downloads page.
Note: there are a handful of build tools which are part of a commercial product, the Desktop Development Environment so you will need to have that in order to do a build.
For detailed build instructions, see How to build RISC OS.