In A Nutshell, What Exactly Is A Cosmic String Fragment?

I recently read (somewhere...I can't remember where...honestly...) that time travel might be easier if we could attach a craft to a Cosmic String Fragment. Since Wormholes are generally accepted as unstable, and black holes tend NOT to travel, this is the best option, since Strings are very stable and do infact travel. It's just a matter of waiting for one to show up. But what do you think? And what exactly IS a CSF anyway?