The average slope (or gradient) gives an indication of how steep the ascending sections of a route are on average. It is calculated as the average of all segments that go uphill — descending or flat sections are therefore not included in this calculation.
This value is always positive (or zero), even if you finish lower than you started, because only the ascending parts of the route are taken into account.
Alternative ways to calculate the average gradient include dividing the net ascent (or the elevation difference = total ascent - total descent) by the distance.