Presence, in the context of mobile phones and/or messaging, refers to technology that allows people to see at a glance whether other people are available to communicate.
Presence is a cornerstone of PC-based instant messaging (IM). It is what allows you too see whether friends in your "buddy list" are "online", "offline", or "away", etc.
On phones, presence is a subset of the Wireless Village instant messaging protocol created by the OMA standards group.
See: Wireless Village
On a phone, presence information (the current status of people you know) can be displayed either in the main contacts list (phone book) and/or within the instant messaging client interface.