One is a suffix that many graffiti writers add to their names to signify that they are the first graffiti writer in a particular area to use that name.
For example, if you were a graffiti writer named "Knife" and you were the first person to write "Knife", in say, Salt Lake City, you would write "Knife-One".
Theoretically, writers will also add a two, three, etc, to their name if their is a graffiti writer that also has their name, but often they will either pick a different name or just claim to be the first. I think, that, perhaps, graffiti writers just like adding "one" to the end of their names.