From The Dhamma Encyclopedia
Zeal (chanda) is a feeling of great energy and enthusiasm. It can be activated by several things – a sense of certainty as a result of studying and practicing the Dhamma, by being inspired by a particularly virtuous or wise person, by experiencing the benefits of meditation, even by contemplating the inevitability of death.
The Buddha says that the value of zeal is that it animates the will and gives us energy, confidence and determination (M.II,175). However, if not held in check zeal can deteriorate into agitation, the mistaken belief that anything can be achieved by sheer will-power, and even fanaticism. We should, the Buddha said, use our energy the way we would catch a quail. If we were to grasp the quail too tight we might squeeze it to death. If we did not hold it tight enough it might fly through our fingers (M.III,159). There needs to be a balance between zeal on the one hand and patience, relaxation and mental calm on the other.