What does the imssege message on Apple's mobile phone mean?

"iMessage is an instant messaging software launched by Apple. When you and your partner are both iphone or ipad, when you have wifi, you don't need to send text messages through telecom merchants, and you don't charge for text messages, just charge traffic. Imessage information means that you and the other party are both iphone or ipad. When you have wifi, you don't send short messages through telecom merchants, you don't charge short message fees, you only charge traffic. IMessage is an instant messaging software launched by Apple, which can not only send text messages, but also send videos, address books, location information, photos and so on. "