What does it mean that Apple 6 plus shows that "your operator may charge for the SMS that activates imessag"?

When sending and receiving iMessage, use mobile phone traffic or WiFi network. If the mobile phone uses iMessage to send and receive messages without opening traffic packets, the operator will charge the mobile phone a traffic fee.

IMessage is an instant messaging software launched by Apple, which can send short messages, videos and so on. It has high security. IMessage is different from SMS/MMS service of operators. Users only need the data support of WiFi or 3G network to complete communication. IMessage uses the new message reminder system of iOS and Mac os, which can directly push information to the other party's screen, regardless of whether the other party is in the game or locked. If both parties are using iMessage, you can even see the state of each other's speech.