What does background_worker in common_site_config.json do

I have installed erpnext using the frappe_docker repo i wanted to know if i increase the background_workers what effect would it have

Depends on if you have a lot of background processing.

It might help if you are running a lot of background jobs in parallel that are “waiting” rather than processing. If you have too many background workers processing, your foreground (web) workers will start getting bottlenecked.