With most English nouns, you simply add an "s" to make the noun plural. The singular word for "noun" is "noun," and if you want to describe more of them, you just add an "s" to make it plural: "nouns." Sometimes there are irregular plurals with no "s," such as the singular "man" becoming the plural "men."
However, there are a number of nouns in English which cannot really be counted and are never used in plural form with an "s" ending. These are called mass nouns (or sometimes "uncountable nouns"). The Oxford Dictionary describes a mass noun as "a noun denoting something that cannot be counted (e.g. a substance or quality), in English usually a noun which lacks a plural in ordinary usage and is not used with the indefinite article."
Children often make the mistake of adding "s" to mass nouns, which is cute but also shows that it's not so easy to understand their usage. "Mommy, look at all the sheeps!" and "I really like to eat spaghettis!" are cute, but unfortunately grammatically incorrect!
You have a sheep and a hippo? -Yeah.
Caption 58, Jimmy Kimmel: Lie DetectivePlay Caption
A herd of bighorn sheep ends up blocking traffic on their way to a better grazing ground.
Caption 1, The Last Paradises: America's National ParksPlay Caption
"Sheep" is a mass noun and there is no such word as "sheeps": One sheep, two sheep, three sheep.
They can play chess, if they wish.Play Caption
The noun "chess" is not used in a plural sense at all. You can have "games of chess," "chess sets," or "chess boards," but in these cases, "chess" is being used as an adjectival describer and the nouns being described are the plurals.
Would you like to sample our vegan bacon?
Caption 1, Parks and Recreation: Best of Ron & AprilPlay Caption
The only way to suggest "bacon" in singular form is to combine it with other descriptions such as "a piece of bacon" or a "little bit of bacon."
All spaghetti spoons have a hole in the bottom.Play Caption
With "spaghetti" too, you can have "lots of spaghetti" but not "spaghettis."
Thank you for showing your support for women's rights.Play Caption
You may see the word "supports" with an "s" in verb form, as in "Prince Harry supports women's rights." You also see it in a different meaning for the noun, such as "the steel supports for the building." But when you are talking about the kind of support that means you are giving approval, comfort, or encouragement to someone or something, then it's always a mass noun.
Go to Yabla English and study the captions in the videos above to get a better idea of the contexts in which mass nouns have been used. You can also go to this site and see a long list of mass nouns.