What does vpn configuration do on my iPhone

The only reason to use a VPN on your phone is that your employer or school requires it to connect to their network.

Despite all the advertising hype, a VPN does not provide any additional security for your iPhone, it actually provides less.

I recommend not using any VPN. It will funnel all internet traffic to the provider of the VPN where many have been caught selling your personal information and it does not offer the protections that they advertise.