The prepend object allows you to concatenate two messages. The argument(s) of prepend will be placed before whatever comes in the inlet (with a space separating the two), and the combined message is then sent out the outlet.
If you want two user interface objects (UI) always to display the same information, it might seem sensible to connect the outlet of one to the inlet of the other and vice versa. However, that will lead to an infinite loop unless you do something to stop one of them from always sending out the value. The set message is useful in that regard, because it allows you to set the contents (and the display) of a UI object without causing that object to send out a message of its own. The top part of the patch demonstrates this technique.