Chromium is an essential trace mineral that makes insulin more efficient.   Chromium repartitions the body by causing glucose disposal into muscle cells instead of fat cells.  What this is saying in laymens terms is that while taking chromium, it will help shuttle sugars and carbohydrates into the muscles as opposed to putting them directly into the fat cells causing you to gain weight that you are not wanting to.  This does not mean to start eating a bunch of bullshit and expect not to get fat.  It just gives you a little more room for error.  Foods such as brewer's yeast or cinnamon are good sources of chromium but most bodybuilders would want   to supplement chromium instead to get a higher dose.  The two forms of chromium that are available are chromium picolinate and chromium polynicotinate.