睛 is the eye 目 plus 青 for sound: the eyeball itself, the wet bright center of seeing. It almost never travels alone — 眼睛 pairs the socket (眼) with the ball (睛).
画龙点睛 'painting the dragon, dotting the eye': the artist Zhang Sengyou left his painted dragons eyeless, warning they'd fly away if finished. Forced to dot two pairs, those dragons burst off the wall. The idiom now means the finishing touch that brings everything to life.
The bright 青 center of the eye 目 — the EYEBALL.
火眼金睛 'fiery eyes, golden pupils' — the Monkey King's see-through-any-disguise vision, earned inside a furnace — is now slang for anyone who spots fakes instantly.