A VLAN lets you run several separate networks over the same cables and switches. For example, staff computers, cameras and guest Wi-Fi can each live in their own VLAN — as if they were physically separate networks.
This improves security: if something happens in one segment (a virus or an unknown guest device), it does not automatically get access to the others. The network also becomes easier to manage and more stable.
Example
In a shop, the checkout, the cameras and the guest Wi-Fi are in three different VLANs — a guest cannot see the checkout network.