On an electric furnace, the heat sequencer lets the heating elements receive electricity in turn rather than all at once. When the furnace first comes on, if all the heating elements could draw electricity at the same time, the resulting spike in current would trip the circuit breakers.
The sequencer lies between the thermostat and the heating elements, which are enclosed metal coils or rods that become hot when electricity passes through them. As the thermostat activates, it sends the electrical current to the sequencer. As the heat builds up, one heating element at a time, the furnace blower circulates the heated air throughout the building.