What was the airline attendants looking at in LED display before light off in A320
The panel you have seen near the forward left door was most probably the Flight Attendant Panel of the CIDS (Cabin Intercommunication Data System). It looks like this: picture of the CIDS panel at jetphotos.com

In the lower part, it has two LED scales which can show the quantities in the fresh and waste water tanks of the aircraft. If I remember correctly, they will only light up while the IND ON pushbutton next to them is pressed.

The cabin crew will routinely check those water levels to ensure e.g. the lavatories can be used.