Evergreen content, which is described as that SEO content which stays relevant for a long time after its initial publication, is a good way to generate income. Comprehensive research statistics and case studies, such as social media marketing trends for the last five years, and detailed how-to guides, such as a beginners guide to using Twitter for business, are always going to be sought after people who wouldn't mind paying small amounts for access to the information.
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.