It's been discovered that a character from the Indian language Telugu can cause iOS's Springboard to crash when sent in a message. The Messages app will just crash over and over when you try to load the app back up, and so to get your Messages app working again you'll have to wait until you get another message and then try to delete the bad thread.
The issue is present in iOS 11.2.5, but has been fixed in the iOS 11.3 beta release. The good news is that Apple won't make us wait until iOS 11.3's spring release for a fix, as it plans to push an update to correct the problem before the full iOS 11.3 update.
Another detail worth noting is that Apple's Messages app isn't the only one affected by this bug. Third-party apps like Facebook Messenger and WhatsApp also have issues with the bug.
Interestingly, this is the second text-related bug that has caused iOS to crash that we've seen lately, with a bug called "chaiOS" affecting devices almost exactly one month ago. All of these bugs are certainly frustrating for iOS users, especially because Apple has been known for having solid software in the past. Recent rumors have said that Apple plans to focus more on ensuring the quality of iOS updates in 2018 rather than just cramming them full of new features, and given all of the recent bugs that've been found lately, we can only hope that that rumor is true.