What does the "two stage" feature of a gas furnace mean to me?                         

A two-stage gas valve adjusts the furnace heating output based on demand. When demand is low the furnace will operate in the lower stage, reducing your fuel bill by about 20%. When the demand is higher the furnace automatically cycles up to the higher stage, providing full capacity and more heat.

The lower operating stage also allows the furnace to heat more evenly with less on/off cycling. Instead of frequent blasts of full capacity heat the furnace will provide just the warmth called for without over-heating your home.