There are many different types of wave. For example we see waves in water. Light is emitted in waves. Sound is also transmitted in waves. One wavelength is a complete cycle of the wave, for example from the mid point up to the top point, through the mid point again and down to the bottom point, then up to the mid point again. That's one cycle of a wave which we call the wavelength. The amplitude is the vertical height between the top point and the mid point. The line across the wave is called the axis.