You can’t lose eco, if you don’t have an eco - Rubenstock
What is an economy, it’s anything that brings you constant resource income: villagers, trade carts, relics, fishing ship and even feitoria.
So what does economy balance mean, it's the amount of excess resources you are getting in. The less excess you have the better your economy is balanced. Size of your economy varies depending on how long the game has been going because there is an absolute limit to the amount of villagers you can have. So when one player has a more efficient eco, he has all the resources incoming being used for either growing his economy or to increase his army numbers.
Ideally players do not bank resources unnecessarily and keeping your resource count low is important. Floating food up slowly as an example is good though, because all age up upgrades do cost food. So while you’re spending all of your resources, you want to be slowly stacking up food and gold for the next age up.
There are a countless number of cases where players at lower levels have 1000 wood unnecessarily, even as early as in feudal age in bank. That 1000 wood does not actually do any good for you. You might think that you can just make a market and sell all of that wood to save the situation, but it’s not quite like that. You end up losing ~30% of your resources when using the market and you’ve to support the excess cost of 175w as well. When you see pros or higher level players doing market stuff in feudal age, they actually know how to balance their eco so that the market is advantageous. Gathering gold is faster than gathering food, so in small amounts buying some food actually gets you to castle age faster at times, it’s not efficient, but the point is not to be efficient, point is to be up faster. So in exchange for a worse economy and lost resources your uptime is accelerated for strategic purposes.
I’d not recommend playing with a market economy, unless you know what you are doing.
So what to do to prevent having too much excess of wrong resources? You keep track of your resources, ideally make new farms each time you have 60 wood extra, unless you need some other building like a blacksmith or something else. In the case of stone and gold, you can simply task villagers to work elsewhere. Finding ways to spend your resources takes a lot of attention and can be very difficult to learn.