AutoVacuumMain

It occurs when the Autovacuum Launcher process is idling between its scheduled searches for tables that need vacuuming or analyzing.
There is a persistent background process in PostgreSQL called the Autovacuum Launcher, which is responsible for periodically checking the database for tables that require maintenance tasks such as vacuuming or analyzing.
When the Launcher has finished checking the database and has spawned all necessary worker processes (up to the limit defined by autovacuum_max_workers), it enters a sleep state. During this sleep period, its wait event is recorded as AutoVacuumMain.