The "User talk:" namespace is the talkspace associated with the User namespace. Pages in this namespace are mainly used to leave messages for particular users or inform them of discussions that may be of interest to them. To facilitate this, when a page User talk:UserName is edited, whenever the user "UserName" loads a page, a notice is displayed at the top of the page informing them of the edit:
You have a new message from another user (last change).
This notice continues to appear until the user loads their user talk page to read the message.
Happy messaging, Kevin
P.S. You may want to check your user page as well!