Yab-yum is generally understood to represent the primordial (or mystical) union of wisdom and compassion. In Buddhism the masculine form is active, representing the compassion and skillful means (upaya) that have to be developed in order to reach enlightenment. The feminine form is passive and represents wisdom (prajna), which is also necessary to enlightenment. United, the figures symbolize the union necessary to overcome the veils of Maya, the false duality of object and subject.
In Tibetan Buddhism, the same ideas are to be found concerning the bell and the dorje, which, like the yab-yum, symbolize the dualism that must be exceeded. The sacred Tantric practice leads to rapid development of mind by using the experience of bliss, non-duality, and ecstasy while in communion with one's consort, either visualized, or in the case of advanced practitioners, in some cases physical. In one an important Anuttarayoga text, where Tilopa expounds the meaning to Naropa, it says:
"When you rely on a consort, the wisdom of empty bliss will arise, so enter into union—the blessing of method and wisdom. Bring it down slowly, retain it, reverse it, and draw it back up. Bring it to the places in the body and let it spread throughout. When you remain free of desire, the wisdom of empty bliss will appear." 
Indicating the advanced nature of the actual practice with consort, the verses are the last in what is already widely considered as a text for the most advanced practitioners, a fact clearly evident in the story about Naropa's receiving the teaching.