When is a cable not "just a cable"? When its insides are designed poorly. Never has that been more of a concern since the USB-C standard came out. By now, many people are familiar with Google's Benson Leung, a man who's made it his personal mission to defend the geek world at large from bad cables - an adventure that just last week managed to kill his Chromebook.
See? That's the real danger of bad cables, and it's for reasons like that why if you know that you're using a bad cable, you should make an effort to procure a good one - especially if you're being offered one for free, which is what Apple is doing right now.
As it happens, the first USB-C cables Apple shipped out are defective, although it's not confirmed what exactly the problem is, or if they pose a real (as in fire) danger. Nonetheless, if you purchased a standalone cable or received one with a notebook through to June 2015, chances are good that your cable is defective.
As seen in the above shot, the later cables have a serial number on them, while the old ones do not. If you have a bunk cable, you have a couple of options: A) Go to an Apple retail store and get the replacement there; B) Go to an Apple reseller; or C) Contact Apple support - the latter of which could just send you on your way to one of the first two.
While you should waste little time in getting your replacement, it is worth noting that you have until June 8, 2018 to really worry about making the swap.