Affiliate and referral marketing involves driving sales for another business which then pays you a commission for each sale it acquires through your efforts. The Amazon Associates affiliate marketing program is one of the best options for people looking to earn money this way. You need set up a website with outbound links redirecting to products on Amazon (or another site that offers affiliate programs) and if a visitor from your site buys the product, then that business pays you a share of the amount spent.
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.