This is a CMOS inverter, a logic gate which converts a high input to low and low to high. Click on the input at left to change its state. When the input is high, the n-MOSFET on the bottom switches on, pulling the output to ground. The p-MOSFET on top switches off. When the input is low, the gate-source voltage on the n-MOSFET is below its threshold, so it switches off, and the p-MOSFET switches on to pull the output high.
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