A chuppah is a tapestry attached to the tops of four poles. The word chuppah is Hebrew for protection or covering, and serves as a roof or covering for the bride and groom during the wedding ceremony.
The chuppah is much more than a ceremonial relic from an ancient past. It serves a clear, though complicated, legal purpose: Standing under the chuppah is the decisive act that formally allows the couple's new state of marriage to take effect, and is the legal conclusion of the marriage process which began with betrothal. Together these two kinyanim (acts of acquisition) are called chuppah v'kiddushin.
Symbolically the chuppah is the parallel to the groom's home, which is to become the bride's new domain, and hints at the bridal chamber, where the marital act was consummated in ancient times.
The wedding canopy that we now know as chuppah was first identified by the Rema (Rabbi Moses Issereles) some 500 years ago, and presumably it was relatively new in his time, but the concept is ancient, and is considered a biblical requirement needed to effect a marriage.
What exactly is chuppah? The mechanics of the act, in halachic terms, is veiled (forgive the pun). In Psalm 19:6 there is a reference to the groom emerging from his chuppah, and Joel 2:16 says, "Let the groom emerge from his chamber and the bride from her chuppah."
According to certain Rishonim, notably the Rif and the Ran, the chuppah ceremony was effected merely by having the bride enter the groom's home. The Rambam held that only through seclusion (yichud) does the chuppah seal the marriage. The Tur maintained that the act of chuppah was achieved by having the groom cover the bride with a garment. Nachalas Shivah cites the Ashkenazi custom that a tallit held over both the the bride and groom's heads was the essential chuppah. Tosafos held that covering the bride's face with a veil finalized the marriage. The Mordechai wrote that when the bride left her father's home and moved into the groom's home was itself chuppah. In light of all these opinions the Bach rules that we perform several of these acts to cover various halachic approaches. The bride is veiled and the canopy serves as the groom's concovering for the bride.
The beautiful Ashkenazi tradition of holding the groom's tallit over the couple's head for the Sheva Brachos (nuptial blessings) has been retained by Sephardim and German Jews.
The bride and groom must stand beneath the chuppah, however nobody else is required to. Usually the rabbi will also be under it as well, and often a cantor (if there is one present), the witnesses and parents will also be under the canopy, depending on its size and the couple's own preferences.
The chuppah – a simple, fragile roof overhead – launches the marriage, teaching them to be content together even with the bare minimum in terms of physical comforts.