The Mapping Provider will be able to map users between the TeamExplorer and any collaboration provider.
The idea is that you will select in the Team-Explorer the mapping Provider which has a configuration(file, sql express, ...) where a im username and the associated collaboration provider is defined.
With this it is available to use more than one collaboration provider at the same time ->
eg 1: skype, icq, msn
eg 2: skype, trillian