"Presence" is the Jabber term for letting other people know that you are online, and additionally how "available" you are. There are three elements to presence: availability status (called "show"), status message, and priority.
Your show status may either be empty (meaning simply "online"), or
one of away, xa, dnd and chat, meaning
"away", "extended away" (i.e. away for an extended period), "do
not disturb", and "free for chat", respectively. This information
is available to everyone subscribing to your presence, but technically
it does not restrict anyone's actions. You can chat with people even
if you claim to be away.
The status message is a short text complementing your show status, such as "at home", "working", "phone", "playing games" or whatever you want. It is sent to everyone subscribing to your presence, but not all clients prominently display it to the user.
The priority is only interesting if you are running more than one Jabber client at a time accessing the same account. In that case, messages sent to you without an indication of which client to send to are sent to the client with the highest priority.
To set your presence, use the function jabber-send-presence.
It can be called both interactively and in Lisp code. For the latter
case, use something like (jabber-send-presence "away" "idle for
10 minutes" 10).
With jabber.el, you can set your presence remotely. Ad-Hoc Commands.