It seems like common sense but it’s so easy to rely on your day job income to pay for everything. I used to get paid a lot of money to go into work and sit at my desk for 8 hours a day and then go home. No manual labor required, no staying late hours(in my case at least) and a pretty low stress environment. I loved my job and without fail, every two weeks on the dot, a nice fat paycheck would show up in my bank account. All I had to do was show up at work every day and I was pretty much guaranteed to get paid.
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.