Graph gaff

Is there a relatively easy way to explain the relationship between the Fs. (impedance?) and the freq. response lines on a graph?  What are we looking for to gain or prevent here?  I am interested only in the frequencies between 70Hz and 4000Hz., so do I want the Fs. to peak below 70Hz?  Does the Fs. peak's relation to being above the freq. line, or below it, make any difference?  I am trying to select a speaker and need more info on this particular.