Away messages act as auto-responders to inbound messages. When an away message is active and a message is received, the contact which sent the message will receive an automatic reply containing the away message as the body of the message.
Away messages utilize the Canned Response library as a source of messages to choose from. In order to set an away message, you must create at least one canned response. Once you have created a canned response, simply hover on the "Available" status indicator to display a list of canned responses to choose from.
To clear your away message, simply hover on the Away Message status indicator and select the "Available" option at the top of the dropdown.