Gil Fronsdal provides a good introduction to equanimity, a quality of balance and flexibility that arises through mindfulness, wisdom, and other helpful qualities. He includes several perspectives on what equanimity is and how to develop it. You can find his article here: Equanimity
Narayan Liebenson discusses equanimity as a response to the Eight Worldly Winds, four pairs of contrasting conditions. We usually desire the positive conditions in each pair, and try to avoid the negative. Those conditions are praise and blame, gain and loss, pleasure and pain, and fame and ill repute. However, we can’t always avoid the negative conditions in our lives, so learning how to find balance when we don’t get what we want, or we get what we don’t want, is a constant exploration. You can find her discussion of equanimity here: Cultivating Equanimity