VLANs are a method of providing Layer2 separation - simplistically creating separate connections within an Ethernet network.
A 'VLAN tag' is applied to the Ethernet frame to identify it.
With the BT FTTC service, there are effectively 2 connections, one (VLAN101) connects the modem to the internet via the ISP.
The other (VLAN301) connects the modem to BT management platform. These 2 connections operate separately from each other.
A search for 'VLAN tagging' or similar will return countless results.
Eats shoots and leaves.