Until this update, it has been possible for a malicious person to get around the lock screen on your iPhone and access your data. This person could do this by making an emergency call and then immediately canceling it. Once the lock screen was bypassed, he could use a USB cable to connect your iPhone to a computer and see your information.
The lock screen is the screen that locks your iPhone and protects it from being accessed by any unauthorized person. After this update, the only way to open your iPhone is to enter your secret numbers.
To see if this update is available for your iPhone, tap Settings, General, Software Update.. If it is, then tap Download and Install after you have connected one end of your USB cable to your iPhone and the other end to a wall outlet.
If you haven't received a notice in Software Update that version 6.1.3 is available (and your iPhone is still eligible for updates), it just means that Apple has yet to send a notice to your iPhone that there is an update available. So just check for it periodically.