What does em heat mean on American Standard Heat Pump System?

"em heat" stands for emergency heat, when the temperature drops below a certain point this will allow your heat strips to kick in, you can also switch at any time to emergency heat and the heat strips will kick in, this however, allows the system to run like you have an electric heat/cool unit vs. a heat pump. The heat pump mode is more efficient but emergency heat will produce warmer air quicker. Hope this answers your question.