Are you running on anything less than iOS 9.3.3? You might want to upgrade. Why?
iOS 9.3.3 comes with bug fixes and security updates (just like almost every update). The update patches malicious code that is able to run on your phone through an image in a text message. Hackers can then access and eventually even possibly control your device. The security flaw was reported by Cisco Talos, a research and security group.
To update your phone, go to your Settings > General > Software Update.
If you need help or have any questions, let us know in a comment below.