If you haven't used a lisp or lisp clone, it's hard to describe fully. Still, the fact that you can trivially replace and reload modules in running code or being able to fix a bug in a function and resubmit the input to the function from the stack trace is amazing ergonomically.

In the same way that most languages have lambdas, it's very different from being able to trivially compose functions into new functions. (This one I can't even describe well, but if you try and write SICP exercises in a lisp vs. python, the difference is obvious.