Well exactly what it says, the scroll speed will be scaled with the bpm.
So with scaling, songs with high bpm will scroll faster and songs with low bpm will scroll slower. In order to maintain the same speed with the scaled option, you'll have to lower your scroll speed on high bpm songs and vis-versa. With the scaling option, speed 20 on 200bpm =/= speed 20 on 150bpm, etc etc.
On the other hand, without scaling, your scroll speed will be (mostly) the same on all songs without the scaling option. Speed 20 on 200bpm == speed 20 on 150bpm, etc etc.