This expression, which originates from the Tanya (Chapter 2), refers to the essential Divine nature of the soul. According to Chassidic thought, every Jewish soul is a literal "part" of Hashem, meaning that its essence is directly connected to the Divine. The term "ממש" (truly) emphasizes that this connection is not just symbolic or metaphorical, but is a real and tangible aspect of the soul.
In Kabbalistic and Chassidic teachings, this concept is central in understanding the relationship between a person and Hashem. The soul, being a part of Hashem, has an inherent purity and sanctity, and the purpose of life is to reveal and manifest this Divine spark through one’s actions and spiritual growth.