Well if we are talking about the same thing, then frequency response is the range of frequencies that a speaker can play. Some speakers can play between ~20hz-~20Khz while others can play ~150hz-~20Khz. If a source (song/movie) sends a signal for a frequency outside of a speakers frequency response, the speaker simply won't be able to play it.
The frequency response of a speaker is how loud a speaker plays at the tones that you can hear. It is usually given as a range of tones or frequencies and the variation of volume either greater of less than the nominal 0 level such as 20 to 20k + or - 5db.
I would not use this as a guide to judge the quality of a speaker without some experience as many makers don't give honest specs and a speaker can be measured incorrectly very easily. It doesn't make sense to not get a speaker you like the sound of because of this measurement
plus let me add the fact that frequency response, does not relate tot hte speakers quality in anyway, but only a very ROUGH idea of its tonal balance, its dynamic range over the frequency range can change the tonal balance a lot.