Curtains means the items in general or as a kind, without specific reference or specified numbers. We need curtains = there are no curtains; any (appropriate) ones will do; nonspecified curtains Lions are dangerous = all lions; any lions; generic lions We are making plans for the holiday = nonspecified plans; all/any (appropriate) plans I have rocks in my pockets = nonspecified rocks Some (= more than one, a sort of plural indefinite article) is often used, where it does not affect the meaning ( I have some rocks, we are making some plans, we need some curtains ). Context and common sense will tell you what is meant.
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