A waveform is the representation of the shape of a wave based on its frequency and amplitude. In electronics, it refers to a signal with a value that changes regularly over time.
An example of a type of waveform is the square wave output by our blinker module that uses a 555 timer. The timer outputs a voltage that is either on or off at a timed frequency, resulting in output that is a square wave. (If you want to learn more, here’s an Instructables about how to make your own 555 timer square wave generator from components.)