Thank you for your question!
Here is an article about it, which was shared here sometime ago. I would recommend you to take a look at it.
Also, there is a good article here http://www.brahmakumarisforum.net/chat/post5841.html#p5841 titled, “Medicine for the diseases of the body and mind is – Mera Baba.” Go through it as well. Towards the end, it will mention about “purity” and its meaning.
This article could be helpful for those of us who are looking for medicines for the soul… 🙂 I believe this article was originally in Hindi and translated by Sister Gayathri from India.