Single stage furnaces are designed to have enough heat output to keep you warm on the coldest day of the year by simply coming on or off. When they operate, they run wide open for a period of time and then shut on and off frequently.
The downside of this design is two-fold. First, all this heating up and shutting down is not efficient as heat is wasted during the cool down period. Second, the single stage furnace produces an up and down temperature swing in your home that is not very comfortable.