What Mussolini meant was that the alliance between Germany and Italy had created had created a bond which the world would revolve around. He was taking a mathematical concept like the Axis of Rotation as a metaphor for the political alliance he fostered. Much like an the earth's axis, Rome and Berlin had created an unshakeable political line between that would force the world to revolve around it, much like the earth's axis in this gif. Mussolini coined the phrase by denying that Italian-German alliance was not the two dictators agreeing to partition Europe, but rather constructing a new "line" which the rest of Europe would orbit. As with many things within fascism, Mussolini's metaphor of the Axis was gigantic puffery- the alliance was far from solid and Germany soon became the leading partner, not an equal- but it also encapsulated fascism's tendency to distill complex geopolitical problems into questions of dominance and subordination.