Sustainable income would be the amount of money that would cover my needs but not necessarily my wants. It will keep our heads above water and makes sure we have a roof over our head and food on the table.
This sounds like a question from a homework assignment. You are wasting your time if you are just asking for answers from others. The whole point is to think for yourself, thus the phrase "in your own words..." Break the question down. What does sustainable mean (long-lasting or environmentally sustainable)? What kind of income is it referring to (individual, business, national)? How do you put those two together? Once you have thought about it you will have your answer, and it will be in your own words. Go for it!