What are noun counters?

Noun counters are words used to express units of uncountable nouns. Some examples are:

The recipe calls for a cup of rice.

Please stop for a bag of flour and a pound of butter on your way home.
She wears a lot of jewelry.
I need some advice on these courses.
The diver checked the gauges on the tanks of oxygen.

Rows of barracks lined each side of the road.