Flip the logic source back and forth and note the NOT gate outputting the inverse bit. LED ON= LOGIC 1/ TRUE. LED OFF= LOGIC 0/ FALSE. When logic is 1 the N channel mosfets gate is saturated allowing the ground to be output as logic 0 while the P channel mosfets gate is off. When logic 0 is input the base of the P channel mosfet becomes saturated and let's voltage conduct through to the output as logic 1.
