When the input is high, the upper PMOS blocks and the lower NMOS conducts. The output is connected to the ground. So the output is low.
An inverter, also called "NOT-gate" consists of one PMOS transistor and one NMOS transistor. (This image also contains two voltage sources, which are not part of the inverter itself, but are connected to it in order to supply it with a dc voltage and to give a pulsed input voltage)
Here the input is the red node and the output is the green node. The PMOS transistor (here the upper one) blocks between Drain and Source, when the input voltage is high. And it conducts, when the input voltage is low. The NMOS transistor (here the lower one) is the other way: It blocks between Drain and Source, when the input voltage is low. And it conducts, when the input voltage is high.
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Title=Digital Circuit 01 Inverter