Can someone explain how prosperity works?

[LordBlade]

Because I'm trying to raise it for one town, but have no real clue what provides the most.

[Tank Lady (Sicilian King)]

Prosperity's the sum of your city's population+science+wealth—one additional level in any category means one more prosperity. Since the threshold for getting a new level in a category goes up as the category's level does, the cheapest way to raise prosperity in an unbalanced town is usually to raise its lowest stat.

(Though if you have powerful synergies in place or could benefit from major bonuses like the Huntress's +100% food yield or a rare micro, you might crank more prosperity from a well-placed bioticum with an already developed resource vs. going into an undeveloped resource—but this is situational.)

[chairman_mentos]

Town prosperity = Sum of town's 3 icons (click on it)
World prosperity = Sum of all town's prosperities. Matters for the Diplomat

[Digus]

Completing request also raise prosperity.