mythlcdserver is a program for displaying MythTV information on an LCD screen through LCDproc. This means you can have data from Myth (channel number, show name, current size of pause buffer,etc., etc.) displayed on an LCD screen. So ... possible to do things like play/advance music tracks without your TV having to be on. This can be enabled in the frontend settings.
Note: if you see messages such as this when running mythlcdserver from a command prompt:
2006-09-04 23:41:32.448 LCDProcClient: WARNING: Something is gettingpassed to LCDd that it doesn't understand 2006-09-04 23:41:32.449 last command: widget_set Time timeWidget 5 2 "11 41 PM"
It means that mythtv has been compiled with a different version of the LCDProc protocol.
MythTV 0.19.0 and prior can only communicate in the LCDProc 0.4.x protocol.
MythTV 0.20.0 and up should be able to communicate in both LCDProc 0.4.x and 0.5.x protocol versions. (this may be a build option?)