A Walkback arises when Squeak cannot find a method for a message it got. It looks thus:
Then what happens is that starts a debugger and tells you it doesn't understand a certain message, while displaying it in its context in the debugger. If you know how to program in Squeak you can rewrite the code; if you are new to Squeak it probably is wise to press 'Abandon' in the Walk-back window. This will restart Squeak at the place it was before it got the message it couldn't process.