Devel is a contributed module that provides a collection of tools for Drupal developers. It is one of the first modules most developers install on their local environment.
It includes handy debugging helpers like dpm for dumping variables, dsm for dumping to the message area, and kint for a nicer rendered variable dump. It also adds a Generate content tool for creating dummy content, plus query logging and menu item access.
Devel is strictly a development tool and should never be enabled on production sites. It can expose sensitive information and has a significant performance impact. Install it on local and staging environments, and rely on it daily for quick debugging. Paired with Kint, it makes Drupal development feel much more interactive than trying to piece things together from logs alone.