Much like a certain three character text message that was crashing phones last year, the link in question (seen in the tweet above) appears innocuous at first glance, since it's a basic GitHub URL. However, simply receiving it can set off a series of problems. According to Twitter user Abraham Masri, who appears to have first pointed it out and has referred to it as the "chaiOS" bug, texting the link will freeze the recipient's device, and possibly restart it.
The folks at 9to5Mac tested it and found it was capable of causing all sorts of problems. They reported that it would sometimes cause both the sender and recipient's Messages apps to crash (on both iOS and macOS) and that it was also causing devices to "respire" (a pseudo restart when the screen displays a spinning circle). They also found that it was causing the so-called "beach ball" to pop up in Safari on macOS, and that there was dramatic lag throughout both iOS and macOS.