What does imessage mean in Apple's mobile phone?

What does imessage mean in Apple's mobile phone? Explain as follows.

Apple's imessage message means to send information through the network. If both parties open the iMessage function and connect to WiFi or cellular mobile network, they can communicate in real time for free. The opening method of iMessage function is as follows: 1. Open the phone settings and find information input 2. After entering the page, just open the button on the right side of iMessage information. 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 Macos, which can directly push the information to the other party's screen, whether the other party is in the game or locked.