Why scientists care
A wave is a propagating disturbance that can transfer energy and information without requiring a net transport of matter over the same distance.
Frequency, wavelength and propagation speed are linked; what oscillates locally depends on the kind of wave and its medium.
A concrete example
If wave speed is fixed, doubling frequency halves wavelength because v = fλ.
Try to notice
Science becomes memorable when a definition is connected to something measurable or observable.