what is VLAN/bridge and should I enable it?

If you open the "Help"on the VLAN/Bridge Group page by clicking on the white arrow (triangle?), it brings up a description.

Essentially, some internet service providers bundle IP television into their package and use a VLAN group to separate the TV data from the "internet" data.  Unless you have one of these services, there is no reason to define it.  As the Help screen points out, the information to configure VLAN comes from the service provider.

As an "aside", some sophisticated networking enthusiasts want to create Virtual Local Area Networks (VLAN) to separate internal traffic.  This is a common feature of industrial and business networks.  Most residentiall WiFi routers do not support VLANs, and this disappoints some customers.