What is actually being displayed is the word "FILt", which is short for filter. The thermostat has a built-in reminder to check/change the HVAC filter. By default this register is set to 10, which is the equivalent of 240 hours of operation.
To clear the reminder, press the "Program" key one time and then press the "Fan" key.
To disable this reminder, you can go into the thermostat setup and change Option 15 to a value of "60".