Overall income potential: Excellent – Although it’s very possible to make $0 with a niche site if it doesn’t receive traffic, you can also make a lot of money if you rank well for good keywords. It’s not likely that you’ll make a living off one site, but they are generally easy to build. If you can build one successfully, you can probably build several more.
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.