I assume you're trying to *cut* the chipboard?
At any rate, to "score" something means to draw (usually a straight line) slightly down into a hard surface (like chipboard or even a sheet of acrylic, etc) with some kind of blade... often an Xacto knife, or utility knife (that type of cutter has a thicker blade). Generally a metal or sturdy ruler is used to guide the blade while scoring.
Doing that breaks the surface of the material exactly along the area you'll want to snap to cut it (or to make more cuts to do the cutting entirely with a blade) to keep the cut "clean" and also gives the material a kind of guideline as to where the cut should be.
Check out these links for info, lessons and more tips on cutting chipboard:
http://www.google.com/search?q=how+to+score+chipboard
P.S. You can also "score" with a blunt tip rather than with a sharp tip like a blade... for example, if you want to fold a piece of cardstock or very thick paper and want the fold to be as neat and sharp as possible, you could use the tip edge of a butter knife or something called a "bone folder" to press and run down the paper where you want the fold to be, then fold right on that line... scoring with a blunt instrument won't cut the surface of the material though, it will just dent it nicely.)
HTH,
Diane B.