When you subsist, you keep yourself alive. Since most airlines have stopped serving food, many travelers are forced to subsist on little packets of pretzels, even on long flights.
To subsist is to feed yourself, or keep yourself going, especially when you just barely manage to do so. A hunger striker might subsist on only water for weeks, and people in developing countries often have to subsist on small rations of rice or grain. Subsist comes from the Latin word subsistere, "to stand firm," but sometimes subsisting makes people feel like they are barely standing, rather than firmly standing.