Financial independence (FI) occurs when investment income is expected to cover your expenses, for the rest of your life.
At this point, work is optional. You can retire, perhaps even early. Or you can continue working as you desire.
The gap between your income and expenses is the fuel behind financial independence. By saving and investing in index funds, you can put your money to work with compound growth.
Once your portfolio reaches your Financial Independence Number, it is expected to provide investment income, adjusted for inflation, for life.