The Divine Masculine
Tantrics see the Divine in all aspects of the Universe. They understand the Universe, and therefore the Divine, in terms of interacting opposites - yin/yang, active/passive, emissive/receptive, masculine/feminine, powerful/motionless, and so on.
The Divine Masculine is a representation of one side of each of the infinite number of pairs of opposites. The Divine Masculine cannot exist without the other poles, so the existence of the Divine Masculine requires, also, the existence of the Divine Feminine.
The separation of the Divine into pairs of opposing forces is the first step of illusion, underlying the creation of all the other layers of illusion which prevent us from perceiving the Universe as it truly is - a unified whole.
The sexual union of the Divine Feminine and the Divine Masculine represents the underlying oneness of the Universe.