This past weekend I had a great Thanksgiving meal with my friends.  As you know, Thanksgiving is when Americans and Canadians make a big feast and eat all day.  Usually people go around the table and say something they are thankful for as well.  This year, we had a great dinner.  Here are some of the things we ate and drank:  roasted turkey, stuffing, ham, wine, gravy, glazed carrots, mashed potatoes, steamed vegetables, crackers, and freshly baked rolls. 
You probably noticed that the first five items on my list are singular nouns, and the second five are plural nouns.  Why is that?  It’s because of our attitude toward the food.  We imagine that some food is like a “substance”, with no shape, while other foods have a shape.  Food that is a substance is considered to be “uncountable”, and food with shape are “countable”.  When we want to talk about how much of something we have on our plate, we can use a lot of different words.  For example, I can say that on my plate I had a lot of turkey and gravy, but not much ham.  Likewise, I ate a lot of mashed potatoes (I love them!), but not many carrots.  When we talk positively, we use “a lot” almost all the time.  Negatively, we use “not much” if the food is uncountable (turkey, ham, wine, or gravy) and “not many” with countable foods (potatoes, carrots, and rolls).
One thing that is very important to remember is that whether or not food is countable/uncountable is very strongly based in culture.  Many foods can actually be both countable and uncountable.  The example of coffee is very interesting.  It used to be that you could only use coffee as an uncountable noun.  For example “Do you want some coffee?”  But ever since coffee culture like Starbucks has taken over, we have the idea in our head of “a coffee”, being the green and white cup with the brown heat sleeve.  So we can now say “I want a coffee”, because we are giving it a shape in our minds. 
If that sounds like too much on your plate for one English meal, do some extra practice at this website:  http://www.engvid.com/english-resource/countable-and-uncountable-nouns/