This equation computes the linear acceleration in the x-direction of a simple harmonic oscillator.
When the restoring force is directly proportional to the displacement from equilibrium, the oscillation is called Simple Harmonic Motion (SHM).
Since the restoring of force of a simple harmonic oscillator is given by Fx=k⋅x [ 1 ] , then also Fx=m⋅ax, and so
ax=Fxm=-k⋅xm
Young, Hugh and Freeman, Roger. University Physics With Modern Physics. Addison-Wesley, 2008. 12th Edition, (ISBN-13: 978-0321500625 ISBN-10: 0321500628 ) Pg 421, eq 13.4