What does it mean that some column icons will appear when iPhone7 holds down the SMS content?

In fact, these are all equivalent to labels. The first red heart means that you have added it to your collection, which is similar to a collection. The second and third are praise and trampling. I believe everyone should know that the third and fourth ones are for greeting, and iMessage could have been regarded as a social software.

IMessage is a cross-device network short message service provided by Apple for iOS. It allows devices such as iPad and iPod Touch that are not bound with mobile phone numbers to communicate with iPhone instantly, and can send text, pictures and videos, which is basically the functional experience of SMS. As long as the iOS device is upgraded to iOS 5 and can access the Internet, you can send and receive SMS for free and unlimited through iMessage. The security of IMessage is very high.