In this context "disclosure" means the explicit acknowledgement of an extraterrestrial presence on earth.
It would ultimately be an act where a government officially and unequivocally announced that earth is being visited by at least one extraterrestrial civilisation.
The term "gradual disclosure" is also used. This often refers to the fact that many countries are gradually declassifying UFO-documents and to the fact that the mainstream media is beginning to cover the UFO-subject in a more serious and nuanced manner.
Some argue that disclosure has already happened in civil society.
Others argue that it would be impossible for an official political system to disclose an extraterrestrial presence without undermining its own sovereignty.