There is a small yellow envelope in the upper right corner of the mobile phone.

The unread message prompt sign is to remind you that there are unread messages in your inbox, and they will disappear when you have finished reading them.

There may also be unsent messages. For example, some fraudulent mobile phones are set not to receive short messages, and sending messages to such mobile phones will prompt that sending failed. With this sign, you can see that the unsent short messages are deleted when you enter the draft or outbox of information.