ReactOS tries to reimplement the Windows NT kernel, and that has always been its Achilles heel, holding it back from a hardware compatibility and stability standpoint. The loss32 concept is to achieve a similar-feeling end result to ReactOS, but built on a more usable foundation, using components known to work well (the Linux kernel, WINE, everything that glues those together, and a sprinkling of ReactOS userland niceties). As a bonus, the OS would still technically be a Linux distro, so it would be possible to run Linux software when necessary, something ReactOS can’t do.
Follow the link and go down to the third point: