Step-by-step explanation:
Not sure if there's a hidden meaning from a book or story that I'm supposed to know...but the most obvious meaning of this is that: cabinet can refer to a group of people (most likely a president's cabinet) that help in making decisions and being involved in the team that aids a leader or organization with advice on any subject matter.
So for this an urgently called secret cabinet meeting, is probably saying that there is a subject matter that needs to be discussed as soon as possible with a leader and his team (cabinet), and is probably secret because the matter at hand is important and can not be disclosed to other people. Hope that helps!