The real answer gets into accounting, and I believe that every investor needs to have at least a basic understanding of accounting principles. Stuff like double entry bookkeeping, debits / credits, expense / capital, and reading financial statements (income statement, balance sheet, cash flow statement) as well as understanding how these are different and why they are essential and complementary for understanding a business.
I also believe there are any number of online primer / education sources for this sort of education, and I expect they'll run between 2 and 8 hours (it's not hard, but it's not trivial). And I've probably freaked out a few accountants along the way (note - I'm not an accountant, nor do I work in anything resembling that field).