"Minting" is what EU2 players have come to call the process of raising money in the treasury via directing monthly income to the "To Treasury" slider on the budget screen. (Saying something like "sending monthly income to the treasury" all the time would just be too long.) "To mint" is a regular verb, having all the normal tenses (minted, will mint, etc.).
If you direct any of your monthly income into the "To Treasury" slider (that is, you mint it), then at the end of each month, that income is turned into money in your treasury. At the same time, you get some inflation. You can see the projected effect of minting on the budget screen, by setting the "To Treasury" slider then mousing over it. The tooltip will show how much income is projected, and how much inflation. Note that the income estimate is rough; in particular it does not project looting effects.
Why did we pick "to mint" as the particular verb to use? The reason for this is that when you mint, you get money (good) but you also get inflation (bad). This combination of effects is vaguely analogous to the economic effect of reminting coins in the real world. Reminting a coin does not have to be dishonest, but it often was. Typically a king would use his monopoly on coinage to lighten the coins, putting slightly smaller amounts of precious metal into each coin when it was reminted. The extra metal would then be taken by the king, minted, and spent, causing inflation of the money supply.
Nowadays, of course, money is not precious metals. Only a tiny fraction of our money is created by actually minting coins, and also the metal used in the coins is not valuable. Most of our money exists in the form of bits in computers; after that, checks and paper money are also far more important forms of money than coins. In the EU2 timeframe, however, there were practically none of those things; money was gold or silver, which was minted into coins.