db is refering to sensitivity. its how loud a speaker plays with a certain amount of power. for instance a speaker with a sensitivity of 92 db should play twice as loud with the same power as one with 89 db. the higher the louder mostly
HZ is frequency. low notes=low numbers, high=high. the human ear can hear from between 20-20,000 hz.(my system does 18-32,000 hz, cool, eh?

) to give you an idea where things are, bass drums are probably around 40-50 hz, singers are around 500-1000 i believe, cymbals probably in the 6000-7000 range, etc.