It's a state of psyhical depletion combined with strong emotional distress caused by performing repetitive intellectual work with not sufficient emotional rewards and not enough variety. Keeping it up for months or years leads to dysregulation and makes the activity harder to harder to perform, until the only option is to avoid performing that activity altogether for months (and sort of debiasing you brain). You can get burnt out on any intellectual task. Think of keeping up an intense physical training exercise without variation in hopes of increasing size of a specific muscle to find instead that you hurt that muscle by overtraining and instead you can't move it anymore.