Pumping is a strategy used to gain speed without pedaling. To pump, you do your best to unweight yourself while manuevering over a hill, and when on the downhill slope, become as heavy as possible. What I mean by this, is you push down when going downhill and lift up over a hill when there is opportunity. This can be applied to berms as well.
This video published by Bike Radar gives a pretty good idea of what it should be like. Check our Need 2 Know Skills Section for more info and videos coming soon!