What's the exclamation point in Apple SMS?

Apple's SMS has an exclamation point because the imessage message has not been opened. The solution is as follows:

Tool: iPhone6s

Operating system: iOS 13.4. 1.

1. In the desktop interface, the information icon has an exclamation point, as shown.

2. Click the Settings icon on the desktop.

3. Open the phone settings interface and click the information option.

4. Open the information interface and click the switch button in the imessage information bar.

5. At this time, the imessage information bar is light green after being successfully opened.

6. But when you quit the desktop, you can see that the exclamation mark on the SMS of Apple's mobile phone has been removed.