Basically, 1 stop = 1 unit of light, so lets say, 2 stops higher, means an increment of 2 units of light
Assume there are 2 settings, 1/500 f/2.8 and 1/250 f/2.8, 1/250 with an aperture of 2.8 would mean double the amount of light entering the light, so that's 1 stop difference.
And btw, to calculate stops, the difference of stops of the aperture is calculated in Root 2