It is the rate of attenuation above the crossover point.
Crossovers are not brick walls. They attenuate the frequencies above (since we are talking about a lowpass crossver) the crossover point at a certain rate, called the slope. The steeper the slope (the higher the number), the more attenuation it applies.
12db & 24db is the rate (slope) of attentuation. It attenuates the frequencies above the crossover point at a rate of 12db/oct, or 12 decibels per octave. The 24db slope attenuates the frequencies above the crossover point at a rate of 24db/oct, or 24 decibels per octave.
So, if you have the crossover point set at 70hz, then with the 12db/oct slope the signal will be down 12db @ 140hz compared to it's level at 70hz. But for the 24db/oct slope, at 140hz the signal would be attenuated by 24db.