A VLAN lets one set of switches and access points carry several isolated networks. Devices on one VLAN cannot talk directly to devices on another unless a router or firewall permits it. Wi-Fi networks are often mapped to VLANs, so the guest network and the office network are kept apart.
VLANs are the building block of network segmentation. Planning them well means deciding which devices belong together, which traffic should be allowed between zones, and documenting the setup so future changes do not accidentally reconnect everything.