Mike, the page that Peter linked demonstrates the meaning of "slope" well and a later page on that site here
shows the different "orders" of audio filters. The number of the order refers to how many basic components are used to form the filter. A first order low pass filter(scroll down to the low pass section)can use simply one inductor(but Gene might be outraged)connected in series which rolls off the high frequencies at 6dB/octave before the woofer. The second order filter adds a capacitor connected in parallel with the woofer(so the two components, an inductor and a capacitor, make it second order)to roll off the high frequencies at 12dB/octave. The parallel connected capacitor has less opposition to the high frequencies than the woofer voice coil, so it "drains off" some some of the high frequencies, increasing the treble roll-off slope. Similarly, the third and fourth order filters shown increase the slope to 18dB/octave and 24dB/octave.
That site has a quite a bit of good audio info with nice graphics. I recall that a few years ago here we used the "Too Little Power" discussion at p.29 of the directory here
to help explain "clipping" and calm some of the exaggerated concern on the point.