The download link provides the latest beta ROM image (a file ‘riscos’) that is required for the Raspberry Pi computer to run RISC OS.
You are advised to start by downloading the full SD card image (RC14) itself (download link to Raspberry Pi foundation web site) (or from the download link just above this one, on this site) which contains all the elements required to run RISC OS, including the firmware to initialise the Raspberry Pi as well as the HardDisc4 boot files, in an SD card image which can be written to a blank SD card.
Once written to the SD card all you have to do is turn on the Raspberry Pi and it will start up in RISC OS with a lot of preloaded software.
Downloading the latest beta development build of the rom image means that you may find some components unfinished. that is what development is about!
If you are new to RISC OS you should first read Introduction to RISC OS
An explanation of how the Raspberry Pi port works is also available.