Every atom in your body is in constant motion. Physics measures that motion as frequency: cycles per second, written in hertz (Hz). Sound is a frequency your ears decode. Light is a frequency your eyes decode. Even the steadiest object at the atomic scale is energy in patterned vibration.
These references start with the basics, what frequency and oscillation actually mean, and go as deep as you want to follow.
The foundation: what a hertz is and how frequency relates to wavelength and period.
Why repeating motion is everywhere: pendulums, springs, atoms, circuits, hearts.
The classic physics lecture on vibration, free to read. Beautiful even if you skim the math.
A visual, clickable map of how vibration works.
How vibrations travel: through air as sound, through space as light. NASA's tour of the spectrum.
The quantum bridge: a particle of light's energy is literally its frequency times a constant.
The U.S. lab where frequency is measured so precisely it defines the second itself.