A gram is a unit of measurement. Some doctors recommend eating one gram of protein for each kilogram of body weight.
Because a gram is a metric unit — in this case, measuring the mass of some object or substance — it isn't used as commonly in the US as it is in Britain and Canada, where metric measurements are standard. Still if you examine an American food label, you'll almost always see the mass in grams listed. The Greek root of gram is gramma, or "small weight."