When you feel lethargic, you're sluggish or lacking energy. Being sleepy or hungry can make anyone lethargic.
Being lethargic makes it hard to get anything done: you feel weak and sleepy. Whatever the reason, a lethargic person needs to snap out of it and get some energy, maybe by eating something or by taking a nap. Being lethargic also goes well with watching TV, since that takes almost no energy at all. When you feel lethargic, you don't have any energy to spare.