What is "slope"???

If you see a plot of freequency v.s. signal when it comes to crossover, at some point which crossover is designed or multiple of such it should have a slope(s).
Since the freequency can't be cut of perfectly vertically there slope defined exponentially.

Expanding just a bit on Marakanetz's response, the slope is the decrease in signal strength per octave. So a first order cross-over has a slope of -3db and it goes up from there -- second is -6, third is -12, fourth is -24. The decrease begins at a certain frequency and keeps on going from there depending on the specifics of the cross-over.

I think 1st is 6 db (decibal) an octave roll-off, 2nd is 12, 3rd is 18, and 4th is 24db an octave roll-off. That's the slope at which the signal is rolling-off With 90 degrees phase shift on each one going up also.