Build a list in a particular niche and tell them stories. Create a bond. Build a relationship with them. It's important. Then, when you've created a bit of culture, start marketing affiliate products or services to them that you think they might like. Just be sure that you personally vet out whatever it is that you're selling to avoid complaints if the product or service falls short.

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.