When you want to clarify something that you just said, you can begin your sentence with "Not that..." For example, if you say that you don't like a certain director's movies, you might want to add that you haven't seen all of his movies:
Not that I've seen all of his movies or anything.
When you say this, listeners will understand that you really meant that you don't like the movies by that director which you have seen.
"Not that..." is useful when you say something that might make other people angry at you, or that they might have misunderstood.
Dude, your girlfriend is absolutely gorgeous. Not that I've been looking at her.