How nice it is if you are making money without directly being involved with it? This type of income comes in a monthly basis so try to imagine yourself receiving huge amount of money in a monthly basis by just sitting in your chair? That's passive income or residual income is.
One way you can achieve this is through rental income. For example, you have large house with some rooms that are not occupied. So why not let other people to rent it and receive a month rental from them? That could be a great source of income and this can add up to your wealth in a monthly basis.