Resonance-The frequency of an applied force

[Woolyabyss]

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

 

[electricspit]

When an object is set into any oscillatory motion it has a frequency, let's say for simplicity, that it 'prefers' to oscillate at. This is its natural frequency. It doesn't have to be in motion to have a natural frequency, but once it is set into non-forced oscillatory motion it will oscillate at its "natural frequency".

Does this clear things up?

Think of a mass-spring system horizontally, if you perturb the system and set it in motion, it will oscillate at its natural frequency after any transients have left the system.

 

[Woolyabyss]

When an object is set into any oscillatory motion it has a frequency, let's say for simplicity, that it 'prefers' to oscillate at. This is its natural frequency. It doesn't have to be in motion to have a natural frequency, but once it is set into non-forced oscillatory motion it will oscillate at its "natural frequency".

Does this clear things up?

Think of a mass-spring system horizontally, if you perturb the system and set it in motion, it will oscillate at its natural frequency after any transients have left the system.

Ya thanks,
I read the definition a bit more closely and realized they weren't saying that the actual force itself had frequency