Tell was the common name for the private messaging system used in the early days of RuneScape. To send an instant message, one would type "tell playername message" in the normal chat area. If the message was unsuccessful in reaching the player, then a reply would be sent back stating so.
The "tell" feature was removed on 14 November 2001. Many players considered "tell" to be useful as it allowed messaging a player without having to add them to their friend's list and allowed players to chat privately without using the mouse.