Oooooohhhh. The economy sliders. Fun.
I'm no expert, but I'll try.
There are three sliders: Tax, Production and Distribution (which is the three sliders for social, military and research).
Tax is the only straightforward one. I'll skip the explanation on that.
The second slider, directly under taxes, is the one that gives most people trouble. This is how much of the money you make is actually used for production / research. Set it to 50%, and only 50% of your theoretical capabilities are used.
I say 'theoretical' because the actual numbers are then dependent on the settings of the three sliders below. The output you determined with taxes and production are shared across the three types according to your settings.
It's an art in itself, but once you understand it, it's workable.
You set your tax slider so you get 100BC surplus (available for distribution).
You set your output slider to 100%, so you get to spend a maximum of 100BC on production.
You set social to 100%.
You then get 100BC worth of social production.
You set social to 50% and research to 50%.
You then get 50BC worth of social and 50BC worth of research.
You adjust your output slider to 50% because you're in econ troubles. You now only get 50BC to spend.
Social at 100% now only nets you 50BC worth of social.
Social at 50% and research at 50%, and you get 25BC social and 25BC research.
The distribution sliders is therefore applied AFTER the production slider, which is dependent on the taxes. Fun.
Of course, once you understand the slider system, there's still focussing production to learn, and why focussing (in DA) is vastly superior to distributing production via the economy screen. But that's a topic for an advanced evening class all in itself.