When a mobile phone provider sends you an update for your phone, it's usually a good idea to install it. Sometimes it's a better idea than others.
This is one of those times: Apple just released an update for iPhones, fixing three very serious bugs that together have been exploited in secret to spy on apparent Middle Eastern targets. Through the flaws, merely clicking on a link can "jailbreak" an iPhone - defeating the security measures Apple has built in and giving the attacker complete control of the device (and any private information on the device).
Your iPhone will prompt you to update to iOS 9.3.5 very shortly. Do it.
Motherboard has an article describing how the flaw was discovered and how it was being used to spy on individuals.
The SANS Internet Storm Center has a concise description of the three flaws and how they work together to compromise a device.
And here is Apple's release bulletin.
What do you need to do?
Open your iPhone or iPad's Settings tool and go to General -> Software Update in your device's Settings app, or connect to iTunes on your Mac or PC. If you are running iOS 9.3.5 (the latest update as of this writing), your device will show that it is up-to-date. If you are running an older version, your device will show an update is available. Install it!