It indicates where the source came from. For example if it was a source that was added by a gedcom you imported it will have "IGED" in it. If you build a source using the normal build routine it will have "new" in it.If you create a new source from within a proof window it will say "quick". There is no need for you to enter anything in that field.
It is helpful if you have set up some sources the quick way, which does not permit you all of the input options as the normal build such as building the bibliography or the other fields under the More button.You can do a source search for all those with quick in the status and then add additional info if necessary. At that time you might want to change the source status to something else to indicated you have edited it.As far as I know you could put anything in there you wanted to, such as "edited".