This simple and most impressive quote is not understood by some of the ordinary people who even passed through this situation. Imagination is the ability of mind to form new mental images, concepts or ideas of external world with his own choices.
Sometimes these images, concepts or ideas are positive while sometimes negative. This positivity or negativity of imagination depends upon the circumstances in the person's external real life.
Positive imagination is about the things that one desire with all the positive aspects in one's mind e.g. a person wants to become a hero/protagonist in some imagination scene. In this state one don't think about how he/she get the desires but only about the gotten desires.
Negative imagination is somewhat different from the negative thinking. Negative imagination is about the bad desires one have in mind just like to become an antagonist in a real life. It is because of some bad effects of society on the person.
Now moving on to the little as an effective quote,"Sometimes imagination sustains life". An example was given to understand the quote. When a person is in state of sorrow or in pain or discouraged by someone and the person can't even say something to others. Then he/she created a world of imagination, he/she makes imaginative characters, concepts and make himself/herself as a retaliator to others who gave pain to him/her. In this way he/she get soothing from that pain to some extent. This is how imagination sustains life of a person even he/she did not understand the quote "Sometimes imagination sustains life"