BgwriterMain

This means the process is currently in its "Main Loop," specifically in a sleep state.

Periodically it wakes up and scans a portion of the shared buffers for dirty pages.
It writes a limited number of them to disk (governed by bgwriter_lru_maxpages) to disk to ensure that the shared buffers do not become too full of dirty pages.
It goes back to sleep for a duration defined by bgwriter_delay