While there are more effective mediums for long-form communication — yes, there is still a thing called email — you might find yourself in the middle of a lengthy iMessage conversation from time to time. Apple is looking to improve that experience in iOS 7, by showing long iMessages in full screen mode.
When you receive a lengthy iMessage on an iPhone, Cult of Mac discovered that it will now truncate the message and display an ellipses with an arrow. You can then tap the conversation bubble and read the entire message in full screen mode. I like to think of it as Reading List for iMessage.