Scala with keyword usage

I found simple example:

class Post extends LongKeyedMapper[Post] with IdPK {
    def getSingleton = Post

    object title extends MappedText(this)
    object text extends MappedText(this)
    object date extends MappedDate(this)
}


object Post extends Post with LongKeyedMetaMapper[Post] {
    def getPosts(startAt: Int, count: Int) = {
        Post.findAll(OrderBy(Post.date, Descending), StartAt(startAt), MaxRows(count))
    }

    def getPostsCount = Post.count
}

What does it mean with IdPK ?

Thanks.