Acceleration of the harmonic oscillator at the time \(t\). This can be, for example, the acceleration of the oscillating mass hanging on a spring.
Maximum deflection of a harmonic oscillator. In the case of a spring pendulum, it is the maximum distance of the mass from the rest position of the spring (undeflected mass).
Time at which the acceleration is \(a(t)\).
Angular frequency \(\omega = 2\pi\,f\) describes how fast the harmonic oscillator oscillates.
Phase is an angular quantity that determines what acceleration \(a(0)\) the harmonic oscillator had at time \(t=0\):$$ a(0) ~=~ -\omega^2\,A\,\cos(\varphi) $$