I'm sorry, but I couldn't possibly explain that sentence about the Beginning in 100 words. As with every bit of Hegel's system, it makes sense only as part of the whole story. This is especially true of the Logic, and even more particularly true of the way it begins. He is not boasting when, in the Introduction to the Science of Logic, he remarks that his method is the only true one. 'This is self-evident simply from the fact that it is not something distinct from its object and content; for it is the inwardness of the content, the dialectic which it possesses within itself, which is the mainspring of its advance.' (Miller translation, p 54.) (So much for the attempt of the 'Marxists' to steal this dialectic from Hegel, and to install it like a second-hand gearbox in the clapped-out old machine called materialism.)
When it comes to the beginning, there is no way to explain it outside the whole shooting-match. Hegel's task was to take the fragmented world which was growing up in Europe after the French Revolution, and show that it was really a unity. But this unity could only be grasped in thought, in a scientific system. 'The True is the Whole.' If you just started with a beginning which was given from the outside, an assumption, an axiom or set of axioms, Aristotle's 'commonly held opinions', or the like, human freedom could not be grasped. God was no longer available for duty as the guarantor of the world. That is why, as he explains, his entire edifice has to be generated by two elements: the beginning and the procedure. The Science must begin without any presuppositions and continue with the only possible method. 'The concept of the Science and therefore the first concept ... must be grasped by the Science itself.' (Smaller Logic, para 17.) We must not start with anything 'mediated', and have to begin with 'pure thought', hence with 'pure being'.
Marx showed that Hegel's Idea was a pseudonym for Capital. (A good place to see this explained is in the interesting book by Hiroshi Uchida, 'Hegel's Logic and Marx's Grundrisse', Routledge, 1985.) Hegel certainly went beyond the ideas of the political economists in some ways, but he could never criticise their standpoint. And one of their unsolved problems was how the whole business started. 'The accumulation of stock must, in the nature of things, be previous to the division of labour', Adam Smith casually remarks. (Wealth of Nations, Introduction to Book 2.) So, somehow, there have to be capitalists with lots of dosh, before capitalist production can get going. Marx knows different. The 'secret of original (ursprünglish) accumulation' is 'a notorious fact that conquest, enslavement, robbery, murder, in short, force, play the greatest part.' Only after he has thus dispelled the mysterious beginning is Marx ready to explain the end of the tale: the socialist revolution.
So Marx's assimilation of Hegel's system, including its beginning, is embodied in his critique of Hegel's dialectical method, 'the direct opposite' of Marx's own. Marx 'begins' with living human beings. Seen from 'the standpoint of human society and social humanity', the inhumanity of modern life, and the potential it contains for true humanity, are both revealed. Hegel's work, on the other hand, is the highest point of their concealment. (If you feel like it, look at the article I appended to chapter 4 of 'Marx at the Millennium'.)