Nodemon options docs don't mention anything with this. Nowhere I can find anything about the option -r.
To give you some context, here is where it is being used.
Update
After answered this question, subsequently, another question popped up: Why use this? There is already an unanswered SO question here
-r is not a nodemon but a node option.
node --help says:
-r, --require=... module to preload (option can be repeated)
In your example, it was "preloading" dotenv-safe/config