The update patches the three previously unknown zero-day vulnerabilities that together are referred to as "Trident".
To check if you're at risk look at Settings > General > About > Version If you're on version 9.3.5 then you're OK. If you're on a different version you should update now. You can pull the update through Settings > General > Software Update then just follow the instructions to download and install it.
For those interested we'll go into the details a little:
The three exploited vulnerabilities are:
- CVE-2016-4655, Memory Corruption in Webkit
- CVE-2016-4656, Information leak in Kernel
- CVE-2016-4657, Kernel Memory corruption leads to Jailbreak.
Lookout and Citizen Lab worked with Apple on the patch before releasing information on the vulnerability.
The Trident vulnerabilities are exploited in a spyware package called Pegasus which is widely available throughout the world