The term "variable speed" refers to the furnace's indoor blower motor, which moves at different speeds to precisely control the flow of heated and cooled air. Better airflow control has several benefits:
Variable speed motors can actually save you money on your energy bills, as they consume less electricity than standard motors.
Variable speed technology also means you will gain heating efficiency or AFUE.
Variable speed motors are excellent for zoning, which allows you to customize your comfort in different areas and control your energy bills.
A variable speed motor can also help clean the air. When the fan is in constant operation (indicated by the "Fan" setting on your thermostat), the motor will continue to slowly circulate air, allowing filters to capture more contaminants.