What is a cc a ml and a mg?

They are units of measurement.

A Milligram or Mg is 1000 micrograms (Usually found in tablets or capsules)
1000 micrograms = 1 milligram, and 1000 milligrams = 1 gram

A Cubic centimeter or CC is a measurement of volume. (Usually found in liquid)
1 cc is equal to 1 Ml

A Millitier or Millilitre or Ml is also a measurement of volume. (Usually found in liquid)
A Ml is a thousandth of a litre
5 ml = 1 teaspoon