I want to replace my old honeywell thermostat LR1620. There are two wires coming out of the wall, but unfortunately the wires are not marked. Can anyone help me?

If you only have a two wire system, one is for r and one is for w. In this case, it doesn't matter if you hook them up wrong. Just hook one to r and one to w, and if it doesn't work, reverse them. Many stats will work with it hooked up either way.