An icon is a being, organization or a concept that is capable of influencing world around PCs by altering fate, using creature-based resources, providing tools, unifying nations, or capturing minds of crowds.
For campaign purposes, from the list of movers and shakers, pick up to seven such entities to be selected by players during character building, and another seven to keep in reserve. For PCs icons are usually sponsors, quest-givers, recurring antagonists too big to take on directly or untrustworthy plotters that seek to take advantage of the heroes. For a GM, an icon is the reason the campaign begins. The unused icons watch the story develop, and can step in when you need the story to make sense, and previously used major NPCs are insufficient to explain going-ons.
EXAMPLE: One of the characters suspects a merchant they have dealings with, to have a sinister secret and breaks in merchants office at night. Now, you were not prepared to develop the merchant further, but deciding to embrace the opportunity you select a suitably dark icon from your reserve list, and from that icon description you extrapolate the dark secret.
When converting a world, it's sufficient to list candidates that fit the definition in the opening paragraph. Then tag them with adequate descriptors (imperials_only, works_from_shadows, inert_for_millennia, etc) and descriptions. Then assign most likely tools and agendas. It's important to pick targets that are different from each other - two spy organizations don't need two different write-ups.
The same icon can be friendly, antagonistic, heroic, tragic, villainous, or manipulative depending on the situation. Icon actions are often unexplainable - let the players decide through their relation current icon role. When describing an icon, leave room for interpretation and ambiguity.
EXAMPLE: Anomander Rake (inescapable destiny, inert for millenia, mystical knowledge). You are a monarch who only sees how things end - the others are shadows and barely perceptible distortions of perception. Since each and any of your actions change the world without affecting overall destiny, you seldom appear to do anything. Your resources are not limited by imagination, but the use you put them to, is always baffling.