With passive income sources, you can go long periods of time without doing work, yet still earn income. In some cases, that income can expand without much additional input from you. For example, if I build a website in 5 hours and after a month, it has only made $50, my time to that point was worth $10/hour. It doesn’t stop there, however. In a year, if the site has earned $600, the value of those initial 5 hours grows to $120/hour. As time goes on, that number can continue to grow indefinitely.
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.