Magokoro(真心) means true heart. But we can translate it as sincerity or honesty with yourself or with others.
In many situations in the dojo or in our daily lives, we fail to learn lessons that could help us evolve (to enlightenment), often for convenience or for the false sense that we already know a certain technique or situation. This happens because we don’t have the habit of questioning ourselves about what we are really able to do. As a result, whenever someone asks us to perform a task or technique we believe we know, only then will we see our shortcomings. Doubt increases by lack of confidence in ourselves. If this happens, it is because you’re not sure about you are able to do.
Ego is a very active agent in this environment. Often, it makes us believe we really know something. And in some cases, it prevents us from learning more about something, as we fear to be seen as inferior to others within the group. This behavior is very common in other environments, beyond the martial one. It favors the emergence of technically and professionally mediocre people, since they can not to deepen their understanding to satisfactorily perform their tasks.
The Kagami (鏡), the little mirror that is exposed at Shinto shrines, serves to reflect yourself the way you really are, with your strong and weak points. If you have an honest heart (magokoro), the Kagami will never lie to you.
Look deep inside your heart.