The economy page is simple. Income is listed on the left and expenses are listed on the right. Primary sources of income are Taxations, Trade, and Tributes, while expenses include army maintenance, fleet maintenance, and tributes.
Each province you control provides a small amount of tax income per month.
Each province has a strategic resource, which provides a small benefit within the province. From one of the province screens, you can click a green trade icon to setup a trade in resources between two provinces.
It is optimal to perform trade between other nations, as it gives more wealth than it does with an internal trade route. However, this might not be possible, as the other nation may be in poor relations, that they may desire a different resource, or if they are in a republic, may be blocked by the senate.
If you receive a trade request, it will appear as an icon at the bottom-left corner of the screen. If ignored, it will be autodeclined.
Tributes may be obtained or paid to other nations. Tributes are established on the diplomacy screen, and is simply a monthly payment to a larger nation for protection.
You can get a steady income from demanding tribute from others, but there's the occasional province that will refuse to provide money. Note that tributaries tend to gradually improve relations over time, as well as impose a stability cost if you declare war against a nation paying you tribute.
Army and Fleet Maintenance
The army and fleet maintenance is the amount of monthly expenses required to keep your military in fit condition. It naturally increases as you increase your army, but becomes more expensive if you exceed the force limit (as shown on the military screen.)
If you want to temporarily reduce military expense, you can do so on the military window. However, this has a drastic impact on morale, and makes your armies easy to defeat in combat.