https://en.wikipedia.org/wiki/Private_VLAN

Private VLAN, also known as port isolation, is a technique in computer networking where a VLAN contains switch ports that are restricted such that they can only communicate with a given "uplink". The restricted ports are called "private ports". Each private VLAN typically contains many private ports, and a single uplink. The uplink will typically be a port (or link aggregation group) connected to a router, firewall, server, provider network, or similar central resource.

This concept was primarily introduced as the number of network segregation (number of vlans) in a Network switch are generally restricted to a specific number and all the resources could be used up in highly scaled scenarios. Hence, there was a requirement to create multiple network segregation with minimum resources.