As I understand it the tax base represents how rich the actual land is. For example, Roma is an exctremely rich city/ province, and thus has a significantly higher tax base than say... any hungarian province. That is because the land itself is inherrintly richer, and will always be so.
You can modify this base income by adding temples, constables, and such. This is you building up the land, making it richer.
Conversely, if you loot the province or burn it, you will be making less money because, while inherrintly rich, you have destroyed what you were taxing.
The tax income is modified by many things. Stability, ideas, advisors, tech (I think) and buildings.
The tax is also separate from production, which makes it's own money, and trade which is obviously different in the interface. I do not understand production much yet, other than that late game it gets huge, and trade is a whole other animal. The provinces will list the trade and production income when you look at them.