Kaddish is a prayer asking that G-d’s name be magnified and sanctified in the world, with a request for the coming of the Redemption and arrival of Moshiach, when the Divine Kingdom will be revealed in its fullness.
One must be very careful to respond to Kaddish, and even make an effort to run to hear it, for its holiness is greater than all other Kedushot (Sanctifications), and responding to it is more important than responding to other Kedushot. It would be proper to respond to at least ten Kaddishes per day. It is forbidden to speak during Kaddish, and severe warnings about this were transmitted by the prophet Eliyahu.
Sources: Tur, Shulcan Aruch and Alter Rebbe, Orach Chaim chapters 55-56. Reprinted from Shoneh Halachot, The Halachic Institute
