A nanometer is 10^{-9} meters.  To get an idea of how incredibly small a nanometer is, we can take a look at the scale of various things we are familiar with.

At the meter scale, there are humans who are about 1 or 2 meters in height.

One thousand times smaller, at the millimeter scale are fleas or ants or the top of a pin. The diameter of hair is several hundred microns or tenths of a millimeter.

Another thousand times smaller, at the micron scale are cells in our body.

Another thousand times smaller, at the nanometer level, are the diameter of DNA.

An atom diameter is a few Angstroms, or a few tenths of a nanometer.

A good example (borrowed from Richard Feynman‘s Lecture on Physics, Volume 1 Chapter 1 Atoms in Motion) is that if you enlarge an apple to the size of the Earth, then the atoms in that apple are now the size of the original apple.

A nanometer is incredibly small!