If you are expert in some area, you can always give your consultancy service to smaller businesses who cant hire big companies. When we were brainstorming with a client of ours to generate second line of income, he identified that he has some great knowledge in the area of terrace gardening. He could see that he can help families with large enough terrace to setup their own balcony garden and earn some fees over weekends or extra time.
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.