In sanskrit, samata means "harmony" or "equanimity".
When I was at my daughter's horse-riding stable the other day, I was drawn to a teenager on her horse, circling around the paddock. Just watching her, it was totally obvious to me what a good horse-rider needs - the balance between mastering her horse (some "push" or action on her part), and yielding her body to the horse, which this girl had attained beautifully. When that balance is struck, everything goes well, the rider and horse are perfectly in sync. 
It always comes back to the play of push and yield: in a yoga pose, and in life: when to yield to what is happening and when to act. That is why for me a total "yin" or passive class or a total "power" or active class doesn't work. There are the two conflicting forces within us - the ying and the yang. We are always balancing these. When we are too passive, we are out of balance and when we are to aggressive we are also. We can start on the mat to observe these two forces within us. In a pose like warrior - are we struggling too much, or collapsing and waiting for it to end? Then we can play with both and find our perfect balance in the pose. Maybe we need to yield to the pose and stop struggling, on the other hand, maybe we need to fire up the "tapas" and be more active. We can only find out which is appropriate by observing and noticing what is going on inside of us and observe if we are moving towards or away from "Samata" when we either push or yield. 