Freelancing is short term project you need to take from someone and complete it in pre-decided time. Once a project is completed you will be paid and there is no further term with the company. There are many websites where people post projects. Many people are working as freelancer full time. Websites like WorknHire or Freelancer are great places to start.
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.