The concept of Summerland is older than Wicca and comes from Theosophy. The earlist reference I can find to it is in a six-volume encyclopedia called The Great Harmonia , written by Andrew Jackson Davis in the eleven-year span between 1850 and 1861. Theosophy itself is a few hundred years older than that and I would imagine that there are indeed earlier references to Summerland... I just can't find them.
According to Theosophy there are several Summerlands and when you die you are sent to one relating to your specific religion, with each religious Summerland being as described in that religion's texts. Their beliefs on Summerland get increasingly complex from there, involving multiple levels of human attainment, relationships to the Astral realm, and specific physical locations in which these Summerlands are supposed to exist, just on a different plane.
The Wiccan version of the Summerland isn't too different. Mention of there being several different ones to cater to different religions is a lot less frequent, but not at all alien. In general it is viewed as a place where all good (or perhaps all) people go to rest and reflect between incarnations. The idea is that we all have specific things we have to learn in life and our time of reflection and contemplation in the Summerland will help us figure out where we need more work and how to address it.
If you'd like to read more about the Theosophic version of Summerland, I really suggest this page; http://users.ez2.net/nick29/theosophy/lessons06.htm
Also worth noting, Wiccans will often call it " the Summerland," whereas Theosophists simply call it "Summerland."