The internet said a typical millionaire has 7 sources of income. I don’t know if this is accurate, but I’m sure wealthy people have more than one source of income. Most of us start off with just one source of income – earned income. That’s how we start our working life. We go to school, get a job, and work hard to get promotions. I still remember the exuberant feeling I got when I saw my first real paycheck. It’s a great thing to work and earn some money. However, to become wealthy, you need to figure out how to generate income when you’re not actively working. That’s what wealthy people do. They let
Investing in rental properties: Another form of real estate investment, rental investments (i.e. becoming a landlord) could steer you down the passive income path of steady monthly rent checks that you can use to pay off a mortgage loan on the rental property. After the mortgage is paid off, those monthly checks go right into your bank account -- potentially for years to come. 

It’s a (mostly) short term, higher risk, higher reward place to invest cash that has a low correlation with the stock market, but is far more passive than buying and managing properties, has more opportunity for diversification than private placements (minimums of 5-10K, rather than 100K), and most of the equity offerings (and all of the debt offerings) provide monthly or quarterly incomes. Unlike a REIT, you can choose exactly which projects you wish to invest in.
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.