ParEdit (paredit.el) is a minor mode for performing structured editing of S-expression data. The typical example of this would be Lisp or Scheme source code.
ParEdit helps keep parentheses balanced and adds many keys for moving S-expressions and moving around in S-expressions.
That quote from EmacsWiki really undersells Paredit, though.
Paredit makes it virtually impossible to un-balance parentheses (aka round, square, and curly brackets).
This mode would be especially interesting for folks avoiding Lisp because of the nightmare of balancing parentheses is too much of an obstacle to overcome (in practice of course it really isn’t, even if you don’t use Paredit).