Kabbalah is the mystical part of Judaism.
The basic concept is that we are all connected to each other spiritually and are here to be kind and help each other by doing as many acts of kindness and charity as we can. It also teaches that everyone is here to learn how to be better through all our life experiences.

You can spend a lifetime learning Kabbalah and never learn all the concepts, but there are some wonderful books on the subject to get you started.
Isaac Luria also called the Arizal and "Ha'ARI Hakadosh" is one of the famous mystics.
The book below is the best book I have read to date on the Arizal - Fascinating