Sam Harris
Truth is, you know what it's like to be perfectly equanimous to feel no craving or aversion or clinging or resistance, to a moment of experience.
Equanimity is associated with the last stage of meditation. It comes from seeing everything in a non-dual way, as mere fabrication, without clinging to or rejecting any experience.
It is sometimes described as "holy indifference", although it is not cynicism or heartlessness. As a characteristic of the awakened mind, it goes along with compassion.
Another way to describe it is through the term "One Taste" : everything that arises has the same taste (eg of "emptiness and liveliness"), rather than being determined by its specific content. Equanimity and One Taste go together.
Pointing out the Great Way
Equanimity is what the mind does. Seen from the point of view of mental events, these events - in Mahamudra view - don´t have a "grab", or stickyness when recognised. They just glide off, rather than rubbing off some traces (which are called "karma" in this tradition).
Resources
Daniel P Brown (Pointing Out)
Brown, D. P., & Thurman, R. (2006). Pointing Out the Great Way: The Stages of Meditation in the Mahamudra Tradition (Annotated ed.). Wisdom Publications.
Rob Burbea (Seeing that Frees)
Burbea, R. (2015). Seeing That Frees: Meditations on Emptiness and Dependent Arising (English Edition) [E-book]. Hermes Amāra. https://www.amazon.de/dp/B00SI7PQD8
This book is praised by Michael Taft, one of the best contemporary teachers, in his "Best meditation books of 2020" https://deconstructingyourself.com/best-meditation-books-2020.html
Bruce Tift (Already Free)
Tift, B., & T. (2015). Tift, B: Already Free: Buddhism Meets Psychotherapy on the Path of Liberation. Sounds True Inc.
https://www.amazon.de/dp/1622034112