This control determines whether the Wireless LAN Controller is configured to prevent clients connected to the same Wireless Local Area Controller from communicating with each other.
Wireless Client Isolation prevents wireless clients from communicating with each other over the RF. Packets that arrive on the wireless interface are forwarded only out the wired interface of an Access Point. One wireless client could potentially compromise another client sharing the same wireless network.
In the context of a WLAN, guest users being able to access other guest user devices typically indicates that Peer-to-Peer (P2P) blocking is not implemented. Enabling P2P blocking will prevent direct communication between connected devices on the same wireless network, which resolves the issue of guest users accessing other guest devices.
References: Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) - Section on WLAN security configurations.