The Rebellion in Hell is an event where a number of demons rebel against the rule of Satan. Ruby is one of the leaders of said rebellion.
After the events of Hell's Broken Angel, a number of demons, including original fallen angel Scarmiglione, decide to forsake their existence as demons and coexist peacefully on earth with humanity. Lucifer forbids this, and in fact tightens his grip as ruler of Hell. This leads to a civil war in Hell, with many demons protesting against Lucifer's strict rule and many others remaining loyal.
(more will be revealed as Hell's Broken Angel progresses.)