"For long-term savings, investing in low-cost index funds is the ultimate passive strategy," Goudreau says. "As legendary investor Warren Buffett recently told CNBC’s On the Money, 'Consistently buy an S&P 500 low-cost index fund. I think it's the thing that makes the most sense practically of all time.' By not picking individual stocks and, instead, buying a low-cost fund that tracks the market, you pay less in fees and take less of a risk. Then you can sit back and watch your money grow over time."
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.