What are the most important economic concepts ?

Cash is King

Two of my faves:

Random Walk Hypothesis

Efficient Market Hypothesis

Never give a sucker an even break

economy is more important wide scale . proper investment is important for good economic growth.

just remember, supply-side economics is really voodoo economics. Daddy Bush was right.

The concept of supply and demand. It's the primary basis of all economies.

Try not to look at a specific course, tool, or concept for most relevance. Rather, the way economics teaches one to look at any given situation is the real value of the discipline. Example of this would be something like studying externalities or a concept like the " Paradox of Thrift". These types of things embody how one should look deeper into things that would have a face value, but through further examination, have a more complex understanding to them. It is the economic "goggles" so to speak that are the value.

Except, economics is an art, and not science. There is no "tool" that can predict future economic activities or results.

Also, there is no way to predict how the stock market will act in the future, because there is no way to predict the world's economic flux.