From the Introduction:
In the Janus Project, the players (with one exception; see "Phase-Free" in the Advantages section) are people called Reactors. They are former residents of Earth who have sacrificed their Earthly forms to bond their consciousness with a denizen of Eden they have a positive reaction with in a process called "phasing."
Once the process is complete, the two personalities merge to become one consciousness with two Phases. The memories of each remain present at all times. This is why the program was title the Janus Project. Two different personalities come together and form one. One Phase is grounded in Eden, the other in Earth. The reactor is someone who is able to swap which Phase is currently dominant.
The main difference between dominant Phases is that the Eden Phase is able to access the magic prevalent in the dimension while the Earth Phase is capable of accessing and commanding the nanites necessary to work current Earth technology which has slowly started being shipped from Earth to Eden.
The purpose of the Reactors is to protect the people of Eden from the Ravagers, members of an Earth group that believes they can forcibly resettle Eden and make it into another Earth, despite this requiring the genocide of the majority of the inhabitants of the dimension.