Q: What does this message mean? An update to the carrier settings for your iPhone is available. Would you like to download it now?

Seriously ?   It is self explanatory.  Do the update from your carrier.

And if that answer isn't satisfactory, you could always contact your carrier.

Yes, this will download updated low-level settings specific to your carrier and how it interacts with your phone. Usually results in improved performance and reliability. Many carriers won't talk to you for tech support until you've installed such an update.

Read through this article to see what this is: iOS: Updating your carrier settings - Apple Support