I've learned this long time ago. Probably early 2000s when there was the debate about joining the EUR.

I thought the basic explanation about money supply was sufficient, but I can try expanding on this.

A typical medium sized country having its own language that has had its own currency for a long time is much more internally economically integrated than externally. We can simplify this by saying huge majority of businesses take part in economic activity within the country, not outside.

This gives us some basis to consider it in separation to it's neighbours even if they are in a single market (smaller countries have less separation, bigger countries more, many factors influence the depth of separation but it exists strongly in countries like Poland).

So this is why it makes sense to talk in terms of money supply in terms of this "island" of economic activity regardless if it has its own currency or shares another.

The we have certain amount of GDP growth each year. More goods and services are produced each year. But simplifying, let's say you and your neighbor are on an island and you have 52 tokens with witch you trade. You catch fish and he works in a field of wheat. You pay him 52 tokens over the course of the year for his wheat and likewise he pays you the same for the fish (one per week for a price of one token).

Suddenly you find a net on the beach and you don't need to catch the fish with a DIY rod. You can catch 5 fish per week rather than 2 of which you sold 1. Now you have 4 fish per week to sell, but the guy only has 1 token per week. So if you want to sell all your fish you have to drop the price to 0.25. You do double the work (the nets are heavy) you get 4x the fish and yet you get the same tokens and wheat. Not fair. So you don't catch more fish.

This is why we need money supply to grow (or be procured by exports, but let's leave this complex topic for now) if the total of goods and services grows too. Ideally at the same rate or slightly more so goods do not depreciate in price.

I already mentioned one reason why deflation is bad, it demotivates producers from growth. Second reason is many costs are static, so it may send them into bankruptcy if others sell more than them at diminished prices. The third reason can also illustrate why we need some inflation.

The inflation is necessary for people to have an incentive to invest their money rather than stuff them into a mattress. If we have deflation it literally makes sense not to spend your money today, because you will get more goods for it tomorrow. Until everyone does this and prices go so low half of business go bankrupt.

So this is why you need money supply to slightly exceed the GDP growth. This subject can be as simple or as complex as you want. But there are basic truths that work on all levels of complexity. This is one of them.