Frequency response analysis is an extension of modal analysis in some way. If you want to know about modal analysis, the full article is here:
http://feaforall.com/whatmodalanaly...

In short, modal analysis helps to determine the modes of vibrations and the frequencies at which those modes are triggered, BUT modal analysis doesn’t give you any info about the real deformation that an excitation of one of those modes will actually cause. When you have to do a dynamic analysis, modal analysis is only the beginning!

Some modes will cause more « resonance » than others (and thus more damage)… so we want to know which ones and we want also to calculate the maximum deformation for each of the modes « triggered ».

Go on the blog to read the full explanation:
http://feaforall.com/frequencyrespon...