A province contributes its income rating to the owner's treasury every turn. Income accumulates in the treasury. The number shown is after all modifications. Income is determined by multiple factors, including population, dominion scales, fortress administration, and unrest.
The basic formula is
If the province has unrest, this number is
Final Income = Modified Income / (1 + (unrest * 0.02)
A "pile of coins" icon displays a general indication of how much income a province can earn. This is simply a graphical approximation. If a province cannot trace an unbroken line of friendly provinces back to a friendly fort, it does not produce income that turn.Taxation requires communication.
Speaking of taxation, note that while the process of income collection is referred to as taxation here, there is no explicit "taxation" mechanism or button in the game. This is a change from previous Dominions games, where you could set the level of taxation in each individual province. This is no longer the case. Taxation back then was not so easy.