Saturation and Cutoff in bipolar junction transistor:
With both of the junctions forward-biased, a BJT is in saturation mode and makes easy high current conduction from the emitter to the collector. This type of mode corresponds to a logical "on", or a closed switch.
In cutoff, biasing conditions that are reverse of saturation (both junctions reverse biased) are present. There is extremely little current that corresponds to a logical "off", or an open switch.