Money is one of the principal resources you have to manage in Victoria 3. This in itself is of course nothing new, but the way money works is a little bit different than what you might be used to, compared to other strategy games.
In most games, money tends to be a resource you accumulate for a specific goal until you have enough of it to achieve that specific goal. For example, you might want to build a building that costs 100 money, and your monthly income is 10 money. That means in order to build the said building, you have to wait for 10 months to accumulate the 100 money needed for the lump sum cost to order the construction of the said building.
The reason for explaining this simple concept is because, in Victoria 3, there is no such thing as a lump sum cost - instead, it’s all about your weekly balance. At the end of every in-game week, your country’s income and expenses are tallied up and the result is then applied to your Gold Reserve or National Debt.
This also means that all forms of expenses, such as construction, also work on a weekly basis - you do not need any cash ‘on hand’ to start construction of a dozen buildings at once, but if you don’t have the revenue to support it you may find yourself quickly going into debt.
Should you be in the positives with your revenue, however, your extra income becomes the Gold reserve of your country. The Gold Reserve is your country’s national stockpile of cash. If you are free of debt, any money that is leftover in the weekly budget after expenses are subtracted is used to increase the Gold Reserve. Conversely, if your expenses exceed your income, this money is taken out of the Gold Reserve to balance the books.
Though it’s certainly never bad in itself to have a sizable Gold Reserve, it isn’t necessarily the best idea to continually run a large budget surplus - each country has a Gold Reserve Limit, which is a soft-cap over which each surplus pound has diminishing returns on the Gold Reserve - if you have an enormous stockpile of gold, a surplus of £10k may only increase your stockpile by as little as £2k, meaning that you’ve simply wasted the rest of your money.