When you tap Reply, a listing is shown of some default replies (canned messages), so that you can quickly reply to a message without typing anything.  You can add/remove these canned messages as needed by selecting Setup | Canned Messages from within a folder.  You can also choose Custom Reply to respond to the message and type whatever you like.

One nice thing about canned messages is that if someone sends you a message from the SkyTel website or using something like a PageWriter, they can setup custom responses that you can use.  For example, my wife can send me a message that says "What do you want for dinner?" with responses of "Pizza", "Chinese", etc.  When I receive the message, I tap Reply to bring up a list of her responses (they will have an * next to them) and select the appropriate response.  You could conceivably have entire conversations with someone without needing to type anything.