"Slope" refers to the angle that frequencies drop in volume between the peak bass area and the upper-midrange. Because low frequencies require much more energy, amplitudes will almost always slope downward. Here are some examples:

The slope is expressed as decibels-per-octave. Typical values are between 3 and 5 dB/octave, although classical and acoustic folk music might be 6 dB/octave while some very bright pop music might be as shallow as 2 dB/octave.
Graphical analyzers such as SPAN let you apply weighting that adjusts for that slope, resulting in the display taking on a more-or-less horizontal appearance. This makes it much easier to compare different spectral ranges, but unless you choose the right weighting it can mess you up.
The idea is to load in a reference track, some recording that's representative of the sound you're shooting for, determine its slope and adjust SPAN's weighting (slope) accordingly. Then you'll have a visual target to reference, independent of monitoring limitations.