Well... how technical do you want... I essentially just spent a term learning about this and all the implications of filters (low pass/high pass/band pass) at a college
level!
Essentially, you can think of it like this... when you have a
capacitor in the
line, it can 'fill up' and 'let out' electrons, but it does this slowly (think low frequencies). By filling up and letting out electrons, it essentially absorbs the low frequencies in your signal. High frequencies however, act quicker then the
capacitor can absorb them so they pass right through.
To see it from abit more mathematical stance... in the frequency domain, the
impedance of a
capacitor is given by 1/(wC) where w is the frequency and C is the capaticance. If you calculate this number for various frequencies you'll notice the the higher the frequency, the smaller the
impedance, so it impedes higher frequencies less then it does low frequencies. This removes low frequencies from the signal before they are sent to the
tweeter!
I'm sure someone else can explain all this much more eloquently then I can, and I can try to explain it in more detail if you'd like.