Resonance-The frequency of an applied force

This isn’t so much homework as it is me having trouble understanding a concept.
I’m having trouble understanding resonance the definition in my book is

"If the frequency of a periodic force applied to a body is the same as or very near to its natural frequency that body will vibrate with very large amplitude. This phenomenon is called resonance."

I don’t understand how a force can have a frequency when its not material.Would it not need to vibrate to have a frequency?
Any help would be appreciated

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