Together with purification and deification, illumination is “the reception of an understanding of the Godhead” (Dionysus the Aeropagite, CH VII.3.209C).
The illumination of conscience is a grace of mercy given by God the Father and a process whereby one sees everything they did or did not do, said or did not say, throughout their life in the light of the truth who is Christ; and all those sins which were not absolved (not recalled by God anymore) through the sacrament of reconciliation. The illumination of conscience is a mini-judgment more popularly known as the life review. It is the second stage of the spiritual life, the three stages being purification, illumination and deification. Illumination is also the beginning of the coming of the kingdom (Mt 6:10, Lk 11:2) in the hearts, souls and minds of mankind.
The aim of the illumination of conscience is that the person receiving it accepts the truth and understanding manifest through it, to reconcile themselves again with God. At no point during the process of illumination, however, is free-will overridden. So the person receiving illumination remains free to accept or reject the grace given, in the latter case rejecting God Himself since He is Truth. The individual illumination of conscience is the process whereby various persons already received or are receiving their life review out of the tender mercy of God. The global illumination of all consciences is the process whereby all mankind will concurrently receive the complete illumination (Jl 3:2) described below. The latter has yet to occur in the history of the world.
Different kinds of illumination of conscience exist. For example, illumination can be partial whereby one sees only the sins committed and which remained unabsolved, with all their consequences to self and others as well as the accompanying anguish. Or illumination can be complete whereby one sees both all of the above and all the good done throughout their life. An example of what illumination can be like is found at the post The Tenderness and Mercy of God the Father in the Illumination of Conscience. Further information about the meaning of illumination of conscience can be found on the page The Global Illumination of Consciences.