Refers to someone one knows personally, not necessarily a close friend. Often used in contrast with strangers or in contexts of mutual recognition.
See also: 知人 (ちじん)
彼は知る人ぞ知る名カメラマンだ。
He is a famous photographer known to those in the know.
知る人もいない街で暮らすのは寂しい。
Living in a town where you know no one is lonely.
Archaic use for a romantic partner. In modern Japanese, 恋人 or 愛人 are more common.
In old writings, there are many poems longing for one's lover.
Phrase composed of the verb 知る (to know) and 人 (person), literally 'a person one knows'. The archaic sense 'lover' derives from classical usage where knowing someone implied intimacy.