Japan wants to take its first steps in the world of particle accelerators. Super KEKB is much smaller than the largest particle accelerator in the world (LHC), 3 km of circumference and in some ways will be able to compete with the LHC but won't be fully operational until 2019.
Depending on the speed at which particles collide with each other in a particle accelerator, some aspects of physics are studied. In the end, these shocks create new particles, so particle accelerators could be called particle "factories".
The energy of moving particles is converted into mass, an aspect of physics we know from Einstein's famous formula (E=mc2), we convert energy into mass using this formula. Higher speed equals more energy and ultimately means that heavier particles can be "made".
LHC is a very large accelerator, specialized in reaching very high speeds and therefore can produce very heavy particles. Super KEKB isn't intended for this, it's a smaller accelerator in which shocks occur at less energy but this has been expressly done to create a type of particle called B meson.
Why is it so important?
These particles were already known and could be our gateway to the next theory in Particle Physics. Right now in Particle Physics we have been working on a very successful theory called the standard model for almost 50 years. This theory generally works very well although it has several small flaws that make scientists suspect that there's something beyond that we aren't able to glimpse.
In the 10 years that LHC has been in operation only particles from the standard model have been discovered, however, in the last four years the LHC seems to indicate that the B meson don't disintegrate as they should, so it may be that nature has chosen the B meson to be the new particle generators and take us to a new level in this field of physics.