Understanding peak frequency and frequency response

Question

I am reading a book about control and I am having trouble understanding a paragraph. It says that the peak frequency should not be too big because then certain frequencies will dominate and the system will become slingy. Could someone give a practical example as to what that means. My understanding is that we must choose a certain frequency we want to our input to have. Am I wrong? Do we look at all the frequencies. I feel I have mixed up stuff in my head.