Answer
Resonance occurs when the frequency of the driving force matches the object's natural frequency. An often-cited example is when you push a swing at its natural frequency, and elicit a large-amplitude response.
Picture the situation: if you push at some other frequency, roughly half the time when you are pushing it, it will be heading toward you, which is not effective at increasing the swing's amplitude.
Work Step by Step
This is discussed on page 382.