A wave is described by three basic properties: wavelength, the distance between successive peaks; frequency, how many peaks pass a fixed point each second; and amplitude, how tall or intense each peak actually is. The wave equation ties the first two of these together with genuine simplicity: wave speed equals wavelength multiplied by frequency, a single relationship that holds for sound waves, light waves and water waves alike, however different the underlying medium each one actually travels through.
Wavelength and frequency trade off against each other to keep speed consistent
For a wave travelling at a fixed speed through a given medium, wavelength and frequency are locked into an inverse relationship: increase one and the other must decrease to keep their product, the wave speed, constant. This is exactly why a higher-frequency sound wave, one with more peaks passing a fixed point each second, necessarily has a shorter wavelength than a lower-frequency one travelling through the same medium, and it's this same trade-off that lets a physicist calculate any one of speed, wavelength or frequency directly from the other two, without ever needing separate, wave-type-specific formulas.
The same equation applies whether the medium is air, water or nothing at all
What makes the wave equation genuinely powerful is that it holds regardless of what's actually doing the waving, air molecules for sound, water molecules for ocean waves, or the electromagnetic field itself for light, which needs no material medium at all. Each medium sets its own characteristic wave speed, sound travels differently through air than through water, but once that speed is known, the same simple wave equation links wavelength and frequency together consistently, which is exactly why the same basic wave mathematics underlies fields as different as acoustics, optics and radio engineering.
What we're still unsure about
That wave speed equals wavelength times frequency, holding consistently across genuinely different physical wave types, is well established, thoroughly confirmed physics taught consistently for well over a century. What's more genuinely a matter of specialised complexity is exactly how this simple relationship holds up in more unusual media where wave speed itself depends on frequency, a phenomenon called dispersion, since in those cases different frequency components of the same wave travel at different speeds and can spread apart over distance, and physicists continue to study and model dispersion's effects carefully for exactly the wave types and media where it matters, rather than treating wave speed as a single fixed number in every situation.
This sits inside Wave Properties & the Wave Equation, one of eight topics in Waves & Optics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.