Ahch-To is the name of the seemingly water world that Rey and Chewbacca found Luke Skywalker on at the end of star Wars - The Force Awakens. It was only shown in the last couple of minutes. It is believed to be the location of one of the first Jedi Temples which he had disappeared in search of. As to whether he found the Temple is open to debate and will most likely be where the adventure starts in Episode VIII when it comes out in 2017. The real world location for Ahch-To is Skellig Michael, an island off the south western coast of Ireland.
In an early trailer for Episode VIII, it looks like the first scenes are set on Ahch-To, and that the film is a direction sequel to what happened in the previous film unlike the previous films where there are gaps between the events..Copyright: Lucasfilm