1. You reload a buggy app in the browser.
2. Oh noes, you hit an error: the browser shows you the stack trace.
3. At the top of the stack trace, there's a "debug" link: if you click that, the HTTP request causes your dev environment to pop up a debugger. No response comes back to the browser yet.
4. In the debugger, it's as if you had a breakpoint right before the error occurred: you can walk back through the stack and inspect the state of each frame, etc.
5. If you see the problem, you can edit the code - right in the debugger UI - and hit save. The stack will unwind to whatever frame you are editing the code in, and restart from the top of that method or block.
6. As execution continues, the HTTP response finally goes out, and you see the (newly fixed) web page come into the browser.
As someone noted in another comment: although I was the main developer and evangelist of Seaside for a while, I no longer use it or contribute to it (though many others have taken up the torch) but I can't tell you how much I miss this workflow.
(Edited for formatting)