I have a Lux 1500 air conditioning system and it has "filter" next to the temp. I just moved into this house so I have no idea what to do and I don't have a manuel on it.

I take it you are referring to the thermostat. It will signal a indicator lite if the filter becomes restricted and needs to be replaced or cleaned depending on the type you have.

If you go to WWW.LUXPRODUCTS.COM/FAQ.HTM This site will give you the complete instructions on how to operate this thermostat. Mike

It means that the filter needs to be changed/