This is extremely handy, because it means you can see at a glance exactly what aspect of your economy you need to develop or where you have surplus production you can exploit. It also means you can see what is costing you money. This is because a production deficit is automatically made up by purchasing the resource you lack. So, if you don't have enough quarries, you can get by as long as you have enough gold. Gold is actually the only resource that you 'spend', though the other resources are constantly consumed by units and structures.
Gold is generated in a number of ways.
The most basic way is through taxes. Each of your towns delivers a basic gold income which grows as the town grows. You can also construct markets and export upgrades to sell off surplus resources for a gold boost. You might also capture a small fortune when you destroy a monster lair. Finally, gold can be gathered from occasional gold deposits you might find on the map.