Devices connecting via encrypted 802.11b network use a WEP key to encrypt the data they send. An actual WEP key looks like: 57EAB0FF for a 40-bit (64-bit) link, and similar, though longer, for a 104-bit (128-bit) link.
However, some 802.11b hardware and software (Apple products, many Linksys, Lucent, D-Link, and other products) have the user input a passphrase instead of an actual WEP key. These products then use some algorithm to convert the passphrase to a WEP key.
The problem Mac users face is that Apples passphrase-to-key algorithm differs from the one used by most other products. Using xwepgen, Apple users can input a Linksys/DLink/Lucent/etc. passphrase, and get the WEP key to use in airport password dialogues.