Income, value, and the basic mechanics of how the money game works.
Money is fundamentally a system for exchanging value. You create value — by doing work, building something, providing a service, creating content, growing a business — and money is the medium through which other people compensate you for that value. Understanding this is more important than understanding any specific financial product, because it reframes the question from 'how do I get money?' to 'how do I create value that others are willing to pay for?'
Most people's first exposure to income is employment: trading time for a fixed hourly rate. That's a completely legitimate and important income type — and understanding how employment income works (gross vs. net pay, taxes withheld, what a W-2 means) is essential adult knowledge. But it's also just one of many: freelance income, business income, investment income, and creator income all work differently and have different risk/reward profiles.
The single most valuable financial concept to understand early is compound growth. Money that grows (through investment, through a business, through savings) grows on top of itself over time. This means that the earlier you understand how money works and start making it work for you, the dramatically greater the outcome. The difference between understanding this at 17 versus 27 is not ten years — it's potentially hundreds of thousands of dollars.