Finances play an important part in Afterwind. You need to have enough money to afford purchasing better units, and be able to support them afterwards. All units cost a certain amount to purchase, and a tenth of that amount per week for maintenance.
Income is generated by your cities and countries. The initial income is 50% for cities on your territory and 10% on foreign territory. The weekly increase is +25% on your territory until 100% and +2% afterwards, up to the total of 150%. Weekly increase on foreign territory is +5%, and the maximum is 50%.
Controlling all cities in the country will also bring you bonus income (you can see it in the Country info menu or in the Finances menu).
Population also influences city income - for example if it falls down to 50%, the nominal income (100%) will be reduced by half.
The difference between your income and expenses is shown in the bottom toolbar, next to the current funds. For more details, look at the Finances menu.