Its a bracelet made of like 12-14 feet of army issued parachute cord. You can just pull two strings on it and it will unravel and you can use the string for what ever you want like to pull something or tie something up. People have used em for many things and i heard it can be used to like pull cars its super strong. And if you buy one from survivalstraps.com then if you use it you show them a pic of it being used and u get another one for free. Also you can get engraved dog tags that can say medical stuff on it like if you get in an accident and your highly allergic to like penicillin you can say that on the bracelet to emergency people will know. They made dog collars out of it and leashes and a bunch of cool stuff.
If you don't know your basic knots, or any survival skills they're beyond fucking stupid to have. Honestly, I could ask almost anyone wearing one to cleat a boat and they would be frozen in confusion, ask them to tie a neuse, or any slipknot and it'll be the same.
Seriously, they're just a trendy thing to have. I mean, they look cool and can be functional, but the majority of people wearing them are only wearing them for the former.