If a gram is divided into one thousand parts, the resultant mass will be one milligram.
"Milli" is a prefix that means "one-thousandth". It is used to form the units of measurement in the metric system that are one-thousandth of a bigger unit. For example, a millimeter is one-thousandth of a meter, a milliliter is one-thousandth of a liter, and a milliAmpere is one-thousandth of an Ampere, the unit of measurement of electric current.
Milligrams are typically used to express the masses of the small amounts of substances in the fields of chemistry, pharmacology, and medicine, among others.
The metric system makes converting between the units easy. Since one milligram is one-thousandth of a gram, to convert from grams to milligrams, multiply by 1000. For example,
56 g = 56*(1000 mg) = 56,000 mg.
Conversely, to convert from milligrams to grams, divide by 1000:
237 mg = 237/1000 g = 0.237 g.