What is a Low Pass Filter

Just as the name implies, the filter will pass all of the frequencies below the cutoff frequency, but will attenuate those frequencies above the cutoff frequency. Generally, the slope of the filter is also specified. If a filter has a slope of 6db per octave, also referred to as a first order slope, and the cutoff frequency is 100HZ then one octave higher, 200HZ the signal has been attenuated by 6db, one octave higher than that, 400Hz, the sound will be down by 12db, and so on.

To answer your second question, attenuate does not simply mean to modify; it specifically means to reduce. The value of reducing frequencies above a given selected frequency is to keep the midrange frequencies out of your woofer. There are several obvious reasons for this. A large cone has a high mass and may not be able to respond quickly enough to produce middle frequencies without many distortions, including those of time and amplitude. The dispersion of a large driver is not sufficient in the middle frequencies, though horn enthusiasts may argue this. Having a small midrange driver and a large woofer covering the same frequencies will surely change the frequency response of the system and having two sources for the same frequency that are not physically close will also create destructive and additive interference between the two wavefronts, just like pitching pebbles into the pond.