The login message prompts the user for a username and password and attempts to login to the current serverside host. The tiddler $:/status/IsLoggedIn reflects the current login status with the values "yes" or "no", and $:/status/UserName reflects the current username.
The login message is handled by the TiddlyWiki core SyncMechanism which invokes the current SyncAdaptorModule (typically the tiddlywiki/tiddlywebadaptor plugin).