A NOT gate is a basic digital logic gate that performs a simple operation: it inverts the input signal. If the input is true (or 1), the output will be false (or 0), and vice versa. This means that the NOT gate produces an output that is the opposite of its input. In digital circuits, NOT gates are essential for creating more complex logic functions. They are often used in combination with other gates, such as AND gates and OR gates, to build circuits that can perform various tasks, including arithmetic operations and data processing.