It means you have to spend close to your entire income to buy enough rice to not starve each day. Basically you are a subsistence farmer or equivalent, you don't have time or energy to do much else than work to not starve. You can't save up to buy a bike or a book, just work for food, all day entire life.

Add a bit more income and suddenly people can afford bikes and books if they save up, that is the lowest point necessary for people to start to pull themselves up and improve their society which is why it is so important to get as many people as possible to at least that level.