There is no particular significance, except to add some extra flair. I have seen similar fringes on state flags so it is not exclusive to the US flag itself.

If you Google it you probably found some conspiracy theorist/sovereign citizen rumors talking about use in wartime or other nonsense, but this is utter malarkey. The fringe is just decorative (and used indoors because it would just wear off very quickly outside).

EDIT: Also, this may be a better question (regarding flag fringes in general) for /r/vexillology