Short Answer: Nothing. Everything. Take your pick Long Answer: Wicca is a religion focused on living. When someone dies, if we mourn, it is for our loss, for the loss of the person to the world, us, their family, and for what they could've done. Few Wiccans mourn the fact that a person has died, only the loss of that person to the Living.
The following are the most common afterlife beliefs held by Wiccans. Our sacred teachings often say nothing of what happens to you after you die. Ours is an EXPERIENTIAL religion.
1.Transmigration of Souls: This belief enters Wicca from Theosophy. It is the belief that life exists as a journey of successive lives, where one learns more, eventually either incarnating as a wise person or becoming one with god. Some Wiccans believe hardships in this life are a result of a mistep on the journey. Others see us as a blank slate at birth.
2. Simple Reincarnation:The soul is born again into a new body, at random.
3. Division of Souls:The soul flies apart at death, where it becomes part of the divine energy of all life, some part of these souls are divided into wisdom given to the living.
4. Non-linear Reincarnation: The soul is born into a new body, with no restrictions on time and space. An individual soul could incarnate in 2001, then die and incarnate in 1857, then die and incarnate in 2001 again. In fact, two people could be the same soul at different stages of incarnation. In otherwords, we could all be Caesar and Cleopatra. NLR may be random, or transmigrative.
5. Familial incarnation:Members of a family reincarnate as their descendants. May be Linear or Non-linear.
6.Choice:Christians go to the Christian afterlife, Hindus reincarnate, and Wiccans go to the afterlife they believe in.
7.Late Hellenic: Souls die and are judged. Some forget the life they lived and reincarnate, others travel to paradise, others to temporary (but often long) torment.
8. Neo-Christian:Good people go to paradise, bad people reincarnate or are ghosts. Good people can reincarnate if they want.
9. What Dreams may Come: People create their afterlife as a reflection of their life and/or death.
10.neoCeltic: People go to a life of everlasting summer and/or beauty and/or youth. (This differs in many ways from what Celtic Reconstructionists believe, and can be said to be a sort of oversimplification of their thoughts on the matter.)
11. You Rot: Your body feeds the worms. Your soul feeds them too.
The above is an example of just a sample of the millions of varied beliefs Wiccans may carry about the afterlife. A form of the above is sometimes proposed by a sect as the belief of that sect, but often it is individual, a belief based on personal research and experience.