To view the preshared key on the ASA platform without copying the configuration file off the box, simply execute: more system:running-config | beg tunnel-group from privilege exec mode.
What is Pre-Shared Key?
According to Wikipedia:
A TKIP passphrase used to protect your network traffic in WPA. A mechanism in Wi-Fi Protected Access (WPA)Personal that allows the use of manually entered keys or passwords to initiate WPA security. The PSK is entered on the access point or home wireless gateway and each PC that is on the Wi-Fi network. After entering the password, Wi-Fi Protected Access automatically takes over. It keeps out eavesdroppers and other unauthorized users by requiring all devices to have the matching password. The password also initiates the encryption process which, in WPA is Temporal Key Integrity Protocol (TKIP) and in WPA2 is Advanced Encryption Standard (WPA2).