Sustain means to support something or keep it going. If you get hungry in the mid-afternoon, you might try snacking to sustain your energy through dinner.
Beams and rafters sustain a roof — they keep it up. If you sustain a conversation over the course of hours, you keep it going during that time. If you sustain an injury, it means you are injured. If you sustain an interest in what you're studying, it means that you're just as excited about the subject after studying as you are when you began.