"That is", when used at the end of a sentence, roughly means "that is to say". We typically use it for clarification or emphasis of something said earlier. Here's an example:
"He was a long-haired kid with freckles; the last time I saw him, that is".
In this example, "that is" is being used to emphasise the fact that my account of him having long hair and freckles only holds true for when I last saw him. In other words, he may not have long hair and freckles now, or at some time prior time, but he did when I last saw him.