Formula Undamped Harmonic Oscillator Frequency Mass Spring constant
$$f ~=~ \frac{1}{2\pi} \sqrt{\frac{D}{m}}$$ $$f ~=~ \frac{1}{2\pi} \sqrt{\frac{D}{m}}$$ $$m ~=~ \frac{D}{(2\pi\,f)^2}$$ $$D ~=~ (2\pi \, f)^2 \, m$$
Frequency
$$ f $$ Unit $$ \mathrm{Hz} = \frac{ 1 }{ \mathrm{s} } $$ Frequency indicates how fast the harmonic oscillator oscillates. For example, how fast a mass on a spring oscillates.
Mass
$$ m $$ Unit $$ \mathrm{kg} $$ Mass of the harmonic oscillator. For a spring pendulum, it is in good approximation the mass hanging on the spring. In the case of a spring whose mass cannot be neglected, a part of the mass of the spring must also be taken into account in \(m\) because it also oscillates.
Spring constant
$$ D $$ Unit $$ \frac{\mathrm N}{\mathrm m} $$ Spring constant describes the stiffness of a spring, i.e. how well the spring can be deflected.