What is the spiritual significance of "Mayim Nukvin" (feminine waters) according to Chassidus? I have seen it plenty of times in Tanya and various Maamarim, but I never really understood the concept.
Another name for it is Isarusa Dilesato אתערותא דלתתא, An arousal of below. It's when we work hard to achieve an inspiration to connect with and serve Hashem. It's called מים נוקבין mayim nukvin because the giver (Hashem) in chassidus is considered male and the recipient (us) is considered female. Our אתערותא דלתתא can cause an אתערותא דלעילה, also called מים דוכרין, male waters, from above. There can also be an אתערותא דלעילה without an אתערותא דלתתא first, but to make it last, you need an אתערותא דלתתא.