While Apple's operating systems are generally viewed as more secure than their (Google or Microsoft-made) counterparts, no such complicated piece of software can ever be truly infallible. The thing that often sets the Cupertino-based tech giant apart from the competition is the way it responds to internal or external discoveries of security issues.
Case in point, it seems that Apple reacted to reports of this latest "vulnerability related to the Walkie-Talkie app" very promptly, disabling the function before the flaw could ever be exploited out in the wild and obviously working on a fix to be deployed as soon as possible. While we don't know the "specific conditions and sequences of events" a potential malicious agent could have employed to exploit the vulnerability against actual users, its nature sounds pretty scary.
Apparently, the flaw could have allowed a skilled enough hacker to "listen through another customer's iPhone without consent." No further details are provided as to how far this invasion of privacy could have gone, but you can probably understand why Apple needed to take such quick, decisive, and extreme action, basically rendering the Walkie-Talkie functionality useless while cooking up a fix.
We could definitely explain how the app works in 100 or so words, but we'd rather leave the talking to Apple's official 2-minute promo video, which covers all the key points in great detail. While somewhat gimmicky, we can absolutely understand why certain users would find Walkie-Talkie useful in certain scenarios when writing a text or making a call on their iPhone is simply not as convenient.
At the end of the day, although iPhone and Apple Watch users have every reason to be annoyed by these discoveries and the temporary deactivation of the Walkie-Talkie app, the important thing to remember is this particular vulnerability was never exploited (at least to Apple's current knowledge) and the functionality will be restored "as soon as possible."