We experience waves all the time, in the form sound and light waves, just to name a few.
In general, these can have any frequency (the pitch for sound) and wavelength. But if we, in some way, confine or constrain the medium producing that wave — such as by fixing the length of a guitar string or setting the length of flute — then only certain wavelengths (and therefore frequencies) are resonant.
For example, when a flute is fingered to produce a ‘C’ note, you cannot blow into it and get a ‘G’ note — although you can blow into it and get a ‘C’ and a octave higher.
In today’s Whys Guys on the WCIA Morning Show, U. of I Professor Paul Kwiat explains more about the phenomenon of confined waves.