choosing a studio monitor

First time purchasing a studio monitors. I have seen that (in the paper) the main difference is the range of frequencies they can play. Examples: 1) Mackie HS624MK2: 45 Hz - 6 Khz 2) Mackie MR5: 60 Hz - 20 Khz (± 3dB) 2) Yamaha HS80M: 42 Hz - 20 Khz (-10 dB) What does this numbers...