A network segment without direct internet access, keeping resources hidden from external traffic. Like a back office that visitors can't access directly - employees can go out through controlled exits, but no one can walk in from the street.
Database servers and backend APIs are placed in private subnets so they can't be directly attacked from the internet, while still communicating with public-facing web servers.
All major clouds use the same concept: a subnet is a slice of a virtual network. A 'private subnet' is a design pattern where the subnet has no direct inbound path from the public internet (no public IPs and/or no route from an Internet Gateway). Outbound internet access, if needed, is provided through controlled egress such as NAT, firewalls, or proxies.