Without having used Elm that much myself: One of its features is its very user friendly error messages. I imagine the author is comparing with more dynamically typed languages, where when you leave your work in the middle of something, there's usually a bit of digging when reopening it and continuing work, which he contrasts with Elm, where the compiler is quite excellent at reporting your unfinished work (in the form of helpful type errors), allowing you to quickly grasp at what point in your program you were at.