A nanogram is one-billionth of a gram. Which means that it is one-thousands of a microgram, and one-millionth of a milligram.
Although knowledge of metric prefixes is enough to ascertain the above information, and further powers of ten can show other equivalences of nanograms, the large scale of the numbers might start to become numbing.
There are very few substances that are regularly calculated in terms of nanograms, although I have seen things expressed in terms of nanograms per milliliter. While there are a few substances whose biological active dose is measured in micrograms, such as LSD, Halcion or Vitamin B12, the only only substance I know regularly measured in nanograms is botulism toxin, whose lethal dose is somewhere around 200 nanograms. For a startling example of just how small a nanogram is, the standard dose of ibuprofen is 200 milligrams, which is equivalent to the amount of botulism toxin needed to kill one million people. A nanogram is a vanishingly small amount, and one that is not often thought about.
However, I am sure that in other fields of endeavor, such as designing computer chips or discussing the life of the Water Bear, the nanogram is used often. However, for most of us, the nanogram is as small in interest as it is in mass.