Wally World is a nickname for Wal-Mart. Usually, i think these refer to an RC-Car type battery, which can be easily made into a rechargeable battery pack for many different preamps, A/D's, or recorders. It just saves you the trouble (and money
) of having to buy tons of double A's and 9-volts every time you want to tape a show. In addition to being rechargable, the battery pack will have a longer life in between charges than a standard battery normally would.
Go to the remote power forum and poke around. You'll find everything you ever wanted to know about building battery packs...and then some.