IPA: /ˌɔːtəʊbaɪˈɒɡrəfi/
KK: /ɔːtəˈbaɪəɡrəfi/
A book or written work about a person's life, written by that person themselves.
She published her autobiography last year, sharing her life story with the world.
Autobiography is formed from "auto-" (meaning self) and "biographia" (from Greek "bios" meaning life and "graphia" meaning writing). The word refers to the writing of one's own life story.
Think of 'self' ('auto-') writing about their 'life' ('bios') — that's what an autobiography is.