Forced harmonic motion occurs when an external force drives a system to oscillate at a specific frequency, rather than allowing it to oscillate at its natural frequency. This can be seen in systems like a swing being pushed at regular intervals, where the push represents the external force that maintains the motion. In this type of motion, the system responds to the external force, leading to a steady-state oscillation. The frequency of the oscillation matches the frequency of the applied force, which can be observed in various applications, such as in mechanical systems and electrical circuits.