It covers Rental Income (Real Estate) or Interest Income (Bank Deposits). Though it is best suited for retirement planning but it can also act as second income during working years. Big question is how to fix the target for Second Income. Very simple, if your Monthly Expense is 50k & residential property can yield rent of 25k then you need 3 residential units i.e. one for self and two for second income source.
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.