This simply means sanding
while the body filler
isn't 100% set up yet. It is important to start green-stage sanding, especially if you have a lot of shaping to do. You can look at another discussion on green-stage sanding in the useful links section.
If you are sanding the filler while it is rubbery and your paper keeps loading up, use a big wire brush to unload it. Helps to make the paper go 5 times longer!