"Calm ... plays a crucial and powerful role on the path of liberation, yet it is often overlooked or underemphasized in our practice. ... It is the soothing factor of mind that quiets the disturbances. It manifests as peacefulness or coolness in both the mind & body. It is what a tired worker feels upon sitting down in the cool shade of a tree on a sweltering day, or what a child feels when her mother lays a cool, soothing hand on her feverish forehead.
Passaddhi (Pali term for calm) encompasses both physical composure & mental tranquility. It is this quality of calm that keeps the mind composed & unruffled in times of difficulty. Buddhist psychology describes how it brings along with it other wholesome states, such as lightness, wieldiness, proficiency, and sincerity. While the first three of these associated states seem obvious concomitants of calm, it is interesting to reflect on the last. Why does calm bring sincerity? When our minds are tranquil, a natural genuineness, honesty, and freedom from duplicity are also present.
In meditation teachings, we sometimes hear of the danger of becoming attached to this wonderfully calming, peaceful state of mind. When we first touch this space of tranquility in our meditation, there is a profound sense of relaxation, relief, and ease, especially as we contrast it with the speed and distractedness of our daily lives, and with the difficulties and struggles we sometimes experience in practice. The tranquility can be so enticing that we might start practicing only for the calm, becoming attached to it and identified with it, and forget that it, too, is a constructed state. We can easily sink into the enjoyment of it and forget to bring mindfulness to it.
Although this is an important caution, the Buddha clearly emphasized the importance of calm in the list of the awakening factors. The happiness of a tranquil mind plays a key role in the path of awakening. Here, the instructions on the factors of enlightenment, found in the Satipaṭṭhāna Sutta, provide crucial guidance:
If the tranquility awakening factor [or any of the others] is present [in him], one knows, “the tranquility awakening factor is present in me”; if the tranquility awakening factor is not present [in him], one knows “there is no tranquility awakening factor present in me.” One knows how the unarisen tranquility factor can arise, and how the arisen tranquility factor can be perfected by development.
It is mindfulness that knows whether tranquility is present or not. And investigation, energy, & interest — what we might call 'meditative intelligence' — lead onward to its development & fullness, but without becoming identified with or ensnared by it."
Goldstein J. "Mindfulness: A Practical Guide to Awakening." Sounds True, 2013.