It is a small number of patches to the base MRI to enable better use of shared memory for forked processes, on operating systems that use copy-on-write semantics for fork. In particular, this optimization is exploited in the Phusion Passenger product that the same developers created. Phusion Passenger is kind of like mod_wsgi but using different process management.
It seems the developers are also trying to offer a higher level of support than is available for the base MRI. For example, they backported the recent security patches to an older version than the MRI developers were willing to do.
Kinda. It is really similar to mod_wsgi. It can run Ruby applications that implement the Rack interface (there is a Rails/Rack adapter). The Rack interface is just the Ruby port of WSGI, so it can also run any WSGI application (e.g. Django).
