Mutual authentication involves both the client and the authentication server verifying each other's identity before network access is granted. This prevents attackers from spoofing an access point (AP) and luring clients to connect to rogue APs (often used in wireless hijacking or evil twin attacks). When mutual authentication (typically via 802.1X with EAP-TLS) is used, clients will not connect unless they can verify the server certificate, which thwarts hijacking attempts.
References:
CWSP-208 Study Guide, Chapter 4 (Authentication and Access Control)
CWNP E-Learning: 802.1X and EAP Authentication Framework
IEEE 802.1X and WPA2-Enterprise concepts