NOT gate is a logic gate circuit which performs NOT operation on input. Its output logic level is always opposite to input. In other words NOT gate inverts its input. Unlike AND and OR gates NOT gate always has single input.
Figure below shows a NOT gate circuit using resistor and transistor:
From the circuit diagram above we see if input is low (0 volt), the transistor will not be biased in other words the transistor will be at cut-off state. Because of the resistor RL the output will be high. If the input gets sufficient positive voltage transistor will be biased and conduct current, hence output will be low. Both results are in line with NOT operation truth table.
This is a demonstration circuit only. In actual digital electronics circuits we use NOT gate integrated circuits (IC) where several NOT gates are packed in a single IC.