Frequency indicates how fast the harmonic oscillator oscillates. For example, how fast a mass on a spring oscillates.
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 describes the stiffness of a spring, i.e. how well the spring can be deflected.