What does it mean that Apple information has an exclamation point?

There is an exclamation mark on the SMS icon of iPhone, indicating that there is an unsuccessful message to send in the SMS.

1. First, we turn on the phone and click the "Settings" system icon. Click the "Information" option again, as shown in the figure below.

2. Then we open the "iMessage" option on the right, close it and restart it.

3. But to clear the exclamation mark on the message, we need to turn off the Apple mobile phone.

4. Then turn on the Apple phone later. At this point, click the "Information" icon again. Then send unsent messages.