Waveform

The waveform is the shape of a wave as it travels. There are many different waveforms. They are usually a shape which is repeated over and over (a "periodic waveform"). A common waveform is the sine wave. It is normally not possible to see a waveform without some device.
Root mean square.
The amplitude of a waveform may change a lot. Even though it changes, the waveform has a root mean square (rms) value. For example: in the UK, the AC mains supply is a sine wave and has a voltage of 240 V. This is an rms voltage. The actual voltage varies:
The amplitude of the sine wave keeps changing from -339.4 V to +339.4 V.
Root mean square is important. It lets us work out many useful things, like power and heating in a wire.
This table has information about working out the rms for some waveforms.