Most nouns in Korean can stand for either one, or more than one, of the item in question. Without the use of 들, the listener will assume singular or plural as implied by the context, or things can even be left ambiguous.
It is most commonly not omitted
- With nouns relating to people:
친구들하고 이야기 했어요 - I talked to / told my friends.
- When using a demonstrative, like 이, 저, 그.
저 책들은 꼭 읽어봐야죠 - I really have to read those books.
Outside of these situations, when singular/plural is important, but not clear from the context, there are other ways to make the number clear.
e.g. 사과 샀어 - (I) bought (some number of apples).
The listener might assume more than one here, but buying one apple isn't that strange. To make it clear that it's one, we could say
사과 한 개 샀어 - (I) bought one apple.
or to make it clear that a number were bought, we could say
사과 몇 개 샀어 - I bought some apples.
It's not necessary to make the noun agree with the number - In English, you can't say "I bought three apple", but in Korean, 사과 세 개 샀어 is fine. However, it's not wrong to say 사과들 세 개 샀어, just a bit redundant and possibly unnatural. Another example would be 사람들이 많아요 - again, it's fine to use 들 here, but not necessary as 많다 makes it clear that the number is more than one.
With nouns relating to people, this redundancy is less unnatural, so 우리들 (we), 여러분들 (people) are fairly natural. In the latter case, '여러분' is often used to address a crowd, in the sense of 손님 여러분, ladies and gentlemen, so 여러분들 might be more naturally used as the subject or object of a sentence.
As in English, nouns that are uncountable can't take '들'.