Yichud means unification and comes from the root word יחד unified or one. A Yichud is a type of a kavana- special Kabbalistic intention.

Man's role consists in the restoration of the original harmony that was destroyed with the breaking of the Vessels; the unification of the name of G-d.

In other words, according to Kabbalah, our role is to achieve the “Tikun Olam”, rectifying the world through the spiritual intention of binding the spiritual and the physical realities as one.

The Arizal says that our role is to restore the connection between the two facets of G-d the Yud+Hey/ Vav +Hey, the intellectual and emotional parts of G-d’s names thereby uniting the above nature and nature realms.

This reuniting process are known as Yechidim, "Unifications".  A great many yechidim were taught. All involve the meditative contemplation of the letters of the various names of G-d.

Israel, Torah and G‑d are bound together in a complete unity, as the Zohar states...

Lectures On Morning Prayer# 2

The ARI tells us that there are 2 levels within the worlds, the external and the internal dynamics. The external level of the worlds are the Divine Chariot, angels and offanim a lower form of angels. Although the “worlds” are spiritual, there is nevertheless the external framework of operation.

The inner level, the soul of creation is manifested in the form of Nefesh within the world of Asiyah, Ruach within the world of Yetzirah, Neshama within the world of Beriyah. Each one of these levels has as well external and internal realms.

It is known what the Zohar (Vayakhel 211b) says that we must begin a fixing on the realm of action, which include 4 preliminary preparations to access the soul of prayer.

1. Cleansing one’s bodily needs by using the bathroom
2. Donning the Tzizit
3. The Teffilin of the arm
4. The Teffilin of the head