NOT gate also know as a logic inverter.
NOT gate contains only one transistor.
NOT gate inverts a logic input.

In above NOT gate
when 1 is given as input,
the transistor turns "off" and outputs a 0.
If we input a 0 then the transistor would be switched "on"
and the NOT gate would output a 1.

Computer logic is based on mathematical logic called Boolean Algebra.
Truth Tables describe logic circuit operations specifying inputs and outputs.
The truth table for NOT gate is

NOT gates used to change logic state from 0 to 1 or 1 to 0.

Use of NOT gates in IC 7404

IC 7404 is a NOT gate IC. It consists of six inverters that perform logical invert action.

Operation of a logic gate