Inverse proportion is when one value value increases, the other decreases. So in this instance, as the wavelength increases, the frequency will decrease
^^ this. It happens because the wavespeed (through a vacuum) is the same --> 3x10^8 m/s, I think... since wavespeed = frequency * wavelength, if you increase the frequency, the wavelength must decrease in order to have a constant wavespeed and vice versa