This minor mode toggles the display of parens. While they are hidden away, the buffer is read-only, because currently this code does not really add and remove parens – it just makes them invisible. That would be a nice feature, though. At the moment, this minor mode is mainly useful to show lisp code to non-lisp coders afraid of parens.
See also ParenFace.
For a similar hack changing the string “lambda” to a real lambda, see PrettyLambda.
For yet another similar hack that makes the parens less visible, see DimParentheses.