Kleenex is a brand of soft paper for the nose or face. In American English especially, people often say a Kleenex when they mean any tissue at all, even if the box is another brand. That is how a famous name can become the everyday word. You will also hear it as a mass word: some Kleenex. Though actually, it is still a company name, and in careful talk you should not treat every tissue as Kleenex. In Britain, tissue is more common. Younger and older Americans both use it this way; it is not slang, just loose everyday speech. In Taiwan nobody asks for “a Kleenex” at the table — they ask for 面紙.