When you’re spiritually healthy, the part of your brain that’s constantly wondering, “what is this all for” has, if not a sufficient answer, at least enough to go on that you’re not getting pulled into nihilism or existential angst.
Think of bodily health. You can be more or less healthy. We call pro athletes healthy when they’re fully capable of doing their sport. Mental health is when your mind isn’t running down halls of mirrors, destroying relationships and self because of phantasms.
Your mind is stronger than your body. Placebos work because they trick the mind into believing that the body doesn’t need to be ill anymore so the body just heals itself.
Similarly, your will is stronger than your mind. You may not be able to direct every thought you have, but you can direct enough of them to where you can generally choose what do do with your time and energy. If you direct your mind with your will to have certain thoughts regularly, then those thoughts filter down into your body and it will respond.
Spiritual health is the maintenance of your sense of control over your mind and body. You need reasons, deep reasons stemming from your individuality as a person, to do, learn, and grow. You can find those reasons in many different ways. You can watch television or movies, much meaning can be found in stories. You can do meaningful things like help the needy or contribute to a cause. You can even daydream or meditate, and come up with meaning for yourself “out of the blue.”
All of these sources of meaning have the same thing in common, they aim to put you in touch with the transcendent. Touching the transcendent, even just a little bit, generally has the effect of healing your spirit right back up and giving you energy to keep pursuing your truth. And you don’t even have to believe in spiritual reality to do it. Sci-fi and comic books work just fine for the materialist atheist.