Uncountable nouns refer to names of materials (air, sand), abstract qualities, collections, and other things which we see as a collective without clear boundaries, and not separate objects.
We cannot use numbers with uncountable nouns. We do not normally use a/an with uncountable nouns, though there are some exceptions.
- Water, not a water or two waters
- Wool, not a wool or two wools
- Weather, not a weather or two weathers
To verify quantities of nouns (uncountable, or countable) think of it like this: