All damped oscillating systems lose energy and will eventually stop moving.
To maintain regular oscillation, energy must be supplied to the system in the form of an external periodic driver force.
The oscillation of a pendulum can be maintained by a small impulse after each cycle.
Forced oscillations are oscillations that are maintained by the driver force.
The driver force imposes a new frequency on the system. The system therefore takes on the frequency of the driver force, regardless of the original frequency of oscillation.
A spinning electrical motor is an example of a system undergoing forced oscillations. In this case, the driver force causes the motor to spin with a particular frequency, regardless of air resistance.