The update fixes a key vulnerability referred to as Broadpwn, which allows hackers to "execute arbitrary code" — or take over your device — via WiFi chips embedded in the device's main processor. A hacker can target these devices so long as they are in range, according to Nitay Artenstein, a security researcher at Exodus Intelligence who first exposed the flaw.
Fortunately, there's an easy fix with the arrival of the new patch. For iPhone or iPad users, head to the Settings menu and select General. From there, you'll see an option called Software Update. Clicking on it should prompt the installation of iOS 10.3.3.
If you're on a Mac, head to the App Store and select the Updates tab. A prompt for macOS Sierra 10.12.6 should load from there.