This documentation applies only Turris OS 3.x that is no longer present in newly sold routers. The new documentation is located at https://docs.turris.cz/.
VLAN is a logically independent (local) network within one network device. It means you can assign different internal network to different physical ports on your router.
You can, for example, setup separated network for untrusted devices (IoT sensors, Smart TV, …) which should not have access to your normal devices such as computers and mobiles.ed