Plain English Meaning
AUTO in a weather report tells you the report was put together by a machine, not a person. No human checked or added to it.
Full Definition
In a METAR, AUTO is a report modifier indicating the observation was generated entirely by an automated weather station, without human augmentation or correction. It appears immediately after the date/time group when applicable.
Context Anchor
Seen near the beginning of a METAR, the routine aviation weather report pilots use before and during flight planning.
Derivation
AUTO comes from the Greek autos, meaning “self.” In this weather-report context, it points to self-operating equipment: the station reports the weather without a human observer.