One of the most basic tenets of Jewish belief is that there is a World to Come and, at some future date, the dead will be resurrected. Kaddish is actually a prayer that reinforces the understanding that life is transient and that we have to accrue as much spiritual merit in this world. One of the things that the recitation of Kaddish does is that the merits are accredited to the deceased and it helps to elevate the soul in the spiritual realms.
Here is a translation of the Mourner's Kaddish.
Glorified and sanctified be God's great name throughout the world which He has created according to His will. May He establish His kingdom in your lifetime and during your days, and within the life of the entire House of Israel, speedily and soon; and say, Amen.
May His great name be blessed forever and to all eternity.
Blessed and praised, glorified and exalted, extolled and honored, adored and lauded be the name of the Holy One, blessed be He, beyond all the blessings and hymns, praises and consolations that are ever spoken in the world; and say, Amen.
May there be abundant peace from heaven, and life, for us and for all Israel; and say, Amen.
He who creates peace in His celestial heights, may He create peace for us and for all Israel; and say, Amen.
